Illustration: Figure 109Fig. 109. An Embryo Chick of about thirty-six hours viewed from below as a transparent object.FB.the fore-brain or first cerebral vesicle, projecting from the sides of which are seen the optic vesiclesop. A definite head is now constituted, the backward limit of the somatopleure fold being indicated by the faint lineS.O.Around the head are seen the two limbs of the amniotic head-fold: one, the true amniona, closely enveloping the head, the other, the false amniona´, at some distance from it. The head is seen to project beyond the anterior limit of the pellucid area.The splanchnopleure fold extends as far back assp. Along its diverging limbs are seen the conspicuous venous roots of the vitelline veins, uniting to form the hearth, already established by the coalescence of two lateral halves which, continuing forward as the bulbus arteriosusb.a, is lost in the substance of the head just in front of the somatopleure fold.HB.hind-brain;MB.mid-brain;p.v.andv.pl.mesoblastic somites;ch.front end of notochord;mc.posterior part of notochord;e.parietal mesoblast;pl.outline of area pellucida;pv.primitive streak.
Fig. 109. An Embryo Chick of about thirty-six hours viewed from below as a transparent object.FB.the fore-brain or first cerebral vesicle, projecting from the sides of which are seen the optic vesiclesop. A definite head is now constituted, the backward limit of the somatopleure fold being indicated by the faint lineS.O.Around the head are seen the two limbs of the amniotic head-fold: one, the true amniona, closely enveloping the head, the other, the false amniona´, at some distance from it. The head is seen to project beyond the anterior limit of the pellucid area.The splanchnopleure fold extends as far back assp. Along its diverging limbs are seen the conspicuous venous roots of the vitelline veins, uniting to form the hearth, already established by the coalescence of two lateral halves which, continuing forward as the bulbus arteriosusb.a, is lost in the substance of the head just in front of the somatopleure fold.HB.hind-brain;MB.mid-brain;p.v.andv.pl.mesoblastic somites;ch.front end of notochord;mc.posterior part of notochord;e.parietal mesoblast;pl.outline of area pellucida;pv.primitive streak.
A section through the posterior part of the head of an embryo of 30 hours is represented infig. 110. The enlarged part of the neural tube, forming the hind-brain, is shewn at (hb). It is still connected with the epidermis, and at its dorsal border an outgrowth on each side forming the root of the vagus nerve is present (vg). The notochord (ch) is seen below the brain, and below this again the crescentic foregut (al). The commencing heart (ht), formed at this stage of two distinct tubes, is attached to the ventral side of the foregut.
On the dorsal side of the foregut immediately below the notochord isseen a small body (x) formed as a thickening of the hypoblast. This may possibly be a rudiment of the subnotochordal rod of the Ichthyopsida.
In the trunk (fig. 108) the chief point to be noticed is the complete closure of the neural canal, though in the posterior part, where the open sinus rhomboidalis was situated at an earlier stage, there may still be seen a dilatation of the canal (fig. 108,s.r), on each side of which are the tail swellings; while the mesoblastic somites stop short somewhat in front of it. Underneath the neural canal may be seen the notochord (fig. 109,ch) extending into the head, as far as the base of the mid-brain. At the sides of the trunk are seen the mesoblastic somites (p.v), the outer edges of which mark the boundary between the vertebral and lateral plates. A fainter line can be seen marking off the part of the lateral plates which will becomepart of the body-wall, from that which pertains to the yolk-sack.
Illustration: Figure 110Fig. 110. Transverse section through the posterior part of the head of an embryo chick of thirty hours.hb.hind-brain;vg.vagus nerve;ep.epiblast;ch.notochord;x.thickening of hypoblast (possibly a rudiment of the subnotochordal rod);al.throat;ht.heart;pp.body cavity;so.somatic mesoblast;sf.splanchnic mesoblast;hy.hypoblast.
Fig. 110. Transverse section through the posterior part of the head of an embryo chick of thirty hours.hb.hind-brain;vg.vagus nerve;ep.epiblast;ch.notochord;x.thickening of hypoblast (possibly a rudiment of the subnotochordal rod);al.throat;ht.heart;pp.body cavity;so.somatic mesoblast;sf.splanchnic mesoblast;hy.hypoblast.
Illustration: Figure 111Fig. 111. Chick of the third day (54 hours) viewed from underneath as a transparent object.a´.the outer amniotic fold or false amnion. This is very conspicuous around the head, but may also be seen at the tail.a.the true amnion, very closely enveloping the head, and here seen only between the projections of the several cerebral vesicles. It may also be traced at the tail,t.In the embryo of which this is a drawing the head-fold of the amnion reached a little farther backward than the referenceu, but its limit cannot be distinctly seen through the body of the embryo.C.H.cerebral hemisphere;F.B.vesicle of the third ventricle;M.B.mid-brain;H.B.hind-brain;Op.eye;Ot.auditory vesicle.OfV.vitelline veins forming the venous roots of the heart. The trunk on the right hand (left trunk when the embryo is viewed in its natural position from above) receives a large branch, shewn by dotted lines, coming from the anterior portion of the sinus terminalis.Ht.the heart, now completely twisted on itself.Ao.the bulbus arteriosus, the three aortic arches being dimly seen stretching from it across the throat, and uniting into the aorta, still more dimly seen as a curved dark line running along the body. The other curved dark line by its side, ending near the referencey, is the notochordch.About opposite the line of referencexthe aorta divides into two trunks, which running in the line of the somewhat opaque somites on either side, are not clearly seen. Their branches however,Of.a, the vitelline arteries, are conspicuous and are seen to curve round the commencing side-folds.Pv.mesoblastic somites.xis placed at the “point of divergence” of the splanchnopleure folds. The blind foregut begins here and extends about up to neary, the more transparent space marked by that letter is however mainly due to the presence there of investing mass at the base of the brain.xmarks the hind limit of the splanchnopleure folds. The limit of the more transparent somatopleure folds cannot be seen.It will be of course understood that all the body of the embryo above the level of the referencex, is seen through the portion of the yolk-sack (vascular and pellucid area), which has been removed with the embryo from the egg, as well as through the double amniotic fold.The view being from below, whatever is described in the natural position as being to the right appears here to the left, andvice versâ.
Fig. 111. Chick of the third day (54 hours) viewed from underneath as a transparent object.a´.the outer amniotic fold or false amnion. This is very conspicuous around the head, but may also be seen at the tail.a.the true amnion, very closely enveloping the head, and here seen only between the projections of the several cerebral vesicles. It may also be traced at the tail,t.In the embryo of which this is a drawing the head-fold of the amnion reached a little farther backward than the referenceu, but its limit cannot be distinctly seen through the body of the embryo.C.H.cerebral hemisphere;F.B.vesicle of the third ventricle;M.B.mid-brain;H.B.hind-brain;Op.eye;Ot.auditory vesicle.OfV.vitelline veins forming the venous roots of the heart. The trunk on the right hand (left trunk when the embryo is viewed in its natural position from above) receives a large branch, shewn by dotted lines, coming from the anterior portion of the sinus terminalis.Ht.the heart, now completely twisted on itself.Ao.the bulbus arteriosus, the three aortic arches being dimly seen stretching from it across the throat, and uniting into the aorta, still more dimly seen as a curved dark line running along the body. The other curved dark line by its side, ending near the referencey, is the notochordch.About opposite the line of referencexthe aorta divides into two trunks, which running in the line of the somewhat opaque somites on either side, are not clearly seen. Their branches however,Of.a, the vitelline arteries, are conspicuous and are seen to curve round the commencing side-folds.Pv.mesoblastic somites.xis placed at the “point of divergence” of the splanchnopleure folds. The blind foregut begins here and extends about up to neary, the more transparent space marked by that letter is however mainly due to the presence there of investing mass at the base of the brain.xmarks the hind limit of the splanchnopleure folds. The limit of the more transparent somatopleure folds cannot be seen.It will be of course understood that all the body of the embryo above the level of the referencex, is seen through the portion of the yolk-sack (vascular and pellucid area), which has been removed with the embryo from the egg, as well as through the double amniotic fold.The view being from below, whatever is described in the natural position as being to the right appears here to the left, andvice versâ.
During the latter half of the second day, and during the third day, great progress is made in the folding off of theembryo. Both the head- and tail-ends of the embryo become quite distinct, and the side-folds make such considerable progress that the embryo is only connected with the yolk by a broad stalk. This stalk is double, and consists of an inner splanchnic stalk, continuous with the walls of the alimentary canal, and an outer somatic stalk, continuous with the body-walls of the embryo. The somatic stalk is very much wider than the splanchnic. (Comparefig. 121E and F, which may be taken as diagrammatic longitudinal and transverse sections of the embryo on the third day.) A change also takes place in the position of the embryo. Up to the third day it is placed symmetrically, on the yolk, with its ventral face downwards. During this day it turns so as partially to lie on its left side. This rotation affects first the head (fig. 111), but in the course of the fourth day gradually extends to the rest of the body (fig. 118). Coincidently with this change in position the whole embryo undergoes a ventral and somewhat spiral flexure.
During the latter part of the second day and during the third day important changes take place in the head. One of these is the cranial flexure. This, which must not be confounded with the curvature of the body just referred to, commences by the bending downwards of the front part of the head round a point which may be considered as the extreme end either of the notochord or of the alimentary canal.
The cranial flexure progresses rapidly, the front-brain being more and more folded down till, at the end of the third day, it is no longer the first vesicle or fore-brain, but the second cerebral vesicle or mid-brain, which occupies the extreme front of the long axis of the embryo. In fact a straight line through the long axis of the embryo would now pass through the mid-brain instead of, as at the beginning of the second day, through the fore-brain, so completely has the front end of the neural canal been folded over the end of the notochord. The commencement of this cranial flexure gives the body of an embryo of the third day somewhat the appearance of a chemist’s retort, the head of the embryo corresponding to the bulb. On the fourth day the flexure is still greater than on the third, but on the fifth and succeeding days it becomes less obvious.
The anterior part of the fore-brain has now become greatlydilated, and may be distinguished from the posterior part as the unpaired rudiment of the cerebral hemispheres. It soon bulges out laterally into two lobes, which do not however become separated by a median partition till a much later period.
Illustration: Figure 112Fig. 112. Side view of the head of an Embryo Chick of the third day as an opaque object.(Chromic acid preparation.)CH.Cerebral hemispheres;F.B.Vesicle of third ventricle;M.B.Mid-brain;Cb.Cerebellum;H.B.Medulla oblongata;N.Nasal pit;ot.auditory vesicle in the stage of a pit with the opening not yet closed up;op.Optic vesicle, withl.lens andch.f.choroidal fissure. The choroidal fissure, though formed entirely underneath the superficial epiblast, is distinctly visible from the outside.1 F.The first visceral fold; above it is seen a slight indication of the superior maxillary process.2,3,4 F.Second, third and fourth visceral folds, with the visceral clefts between them.
Fig. 112. Side view of the head of an Embryo Chick of the third day as an opaque object.(Chromic acid preparation.)CH.Cerebral hemispheres;F.B.Vesicle of third ventricle;M.B.Mid-brain;Cb.Cerebellum;H.B.Medulla oblongata;N.Nasal pit;ot.auditory vesicle in the stage of a pit with the opening not yet closed up;op.Optic vesicle, withl.lens andch.f.choroidal fissure. The choroidal fissure, though formed entirely underneath the superficial epiblast, is distinctly visible from the outside.1 F.The first visceral fold; above it is seen a slight indication of the superior maxillary process.2,3,4 F.Second, third and fourth visceral folds, with the visceral clefts between them.
Owing to the development of the cerebral rudiment the posterior part of the fore-brain no longer occupies the front position (fig. 111, and112FB), and ceases to be the conspicuous object that it was. Inasmuch as its walls will hereafter be developed into the parts surrounding the so-called third ventricle of the brain, it is known as the vesicle of the third ventricle, or the thalamencephalon.
On the summit of the thalamencephalon there may now be seen a small conical projection, the rudiment of thepineal gland, while the centre of the floor is produced into a funnel-shaped process, the infundibulum, which, stretching towards the extreme end of the alimentary canal, joins the pituitary body.
Beyond an increase in size, which it shares with nearly all parts of the embryo, and the change of position which has already been referred to, the mid-brain undergoes no great alterations during the third day. Its sides will ultimately become developed into the corpora bigemina or optic lobes, its floor will form the crura cerebri, and its cavity will be reduced to the narrow canal known as the iter a tertio ad quartum ventriculum and two diverticula leading from this into the optic lobes.
In the hind-brain, or third cerebral vesicle, the roof of the part which lies nearest to the mid-brain, becomes during the third day marked off from the rest by a slight constriction. This distinction, which becomes much more evident later on bya thickening of the walls and roof of the front portion, separates the hind-brain into the cerebellum and the medulla oblongata (fig. 112CbandHB). While the walls of the cerebellar portion of the hind-brain become very much thickened as well at the roof as at the sides, the roof of the posterior portion or medulla oblongata thins out into a mere membrane, forming a delicate covering to the cavity of the vesicle (fig. 114IV), which here becoming broad and shallow with greatly thickened floor and sides, is known as the fourth ventricle, subsequently overhung by the largely-developed posterior portion of the cerebellum.
Illustration: Figure 113Fig. 113. Head of an Embryo Chick of the fourth day viewed as an opaque object: from the front in A, and from the side in B.(Chromic acid preparation.)CH.cerebral hemispheres;FB.vesicle of the third ventricle;Op.eyeball;nf.nasofrontal process;M.cavity of mouth;SM.superior maxillary process ofF. 1, the first visceral fold (inferior maxillary process);F. 2,F. 3, second and third visceral folds;N.nasal pit;ot.otic vesicle.In order to gain the view here given the neck was cut across between the third and fourth visceral folds. In the sectionethus made, are seen the alimentary canalal, the neural canaln.c., the notochordch, the dorsal aortaAO, and the vertebral veinsV.
Fig. 113. Head of an Embryo Chick of the fourth day viewed as an opaque object: from the front in A, and from the side in B.(Chromic acid preparation.)CH.cerebral hemispheres;FB.vesicle of the third ventricle;Op.eyeball;nf.nasofrontal process;M.cavity of mouth;SM.superior maxillary process ofF. 1, the first visceral fold (inferior maxillary process);F. 2,F. 3, second and third visceral folds;N.nasal pit;ot.otic vesicle.In order to gain the view here given the neck was cut across between the third and fourth visceral folds. In the sectionethus made, are seen the alimentary canalal, the neural canaln.c., the notochordch, the dorsal aortaAO, and the vertebral veinsV.
The third day, therefore, marks the distinct differentiation of the brain into five distinct parts: the cerebral hemispheres, the central masses round the third ventricle, the corpora bigemina, the cerebellum and the medulla oblongata; the original cavity of the neural canal at the same time passing from its temporary division of three single cavities into the permanent arrangement of a series of connected ventricles,viz.the lateral ventricles, thethird ventricle, the iter (with a prolongation into the optic lobe on each side), and the fourth ventricle.
By the third day the lens of the eye has become formed by an invagination of the epiblast, and other changes in the eye have taken place. The external opening of the auditory pit is closed before the completion of the third day (fig. 114,RL); and the rudiments of the external parts of the organ of smell have become formed as small pits on the under surface of the fore-brain (fig. 112,N). Like the lens and the labyrinth of the ear, they are formed as invaginations of the external epiblast; unlike them they are never closed up.
Illustration: Figure 114Fig. 114. Section through the hind-brain of a Chick at the end of the third day of incubation.IV.Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle.Ch.Notochord;CV.Anterior cardinal vein;CC.Involuted auditory vesicle;CCpoints to the end which will form the cochlear canal;RL.Recessus labyrinthi (remains of passage connecting the vesicle with the exterior);hy.Hypoblast lining the alimentary canal;AO.,AOA.Aorta, and aortic arch.
Fig. 114. Section through the hind-brain of a Chick at the end of the third day of incubation.IV.Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle.Ch.Notochord;CV.Anterior cardinal vein;CC.Involuted auditory vesicle;CCpoints to the end which will form the cochlear canal;RL.Recessus labyrinthi (remains of passage connecting the vesicle with the exterior);hy.Hypoblast lining the alimentary canal;AO.,AOA.Aorta, and aortic arch.
During the second and third days there are formed the visceral or branchial clefts, homologous with those of the Ichthyopsida, though never developing branchial processes from their walls.
They are however real clefts or slits passing right through the walls of the throat, and are placed in series on either sideacross the axis of the alimentary canal, lying not quite at right angles to that axis nor parallel to each other, but converging somewhat to the middle of the throat in front (fig. 112andfig. 113).
Four in number on either side, the anterior is the first to be formed, the other three following in succession. They originate as pouches of the hypoblast, which meet the epiblast. At the junction of the epiblast and hypoblast an absorption of the tissue is effected, placing the pouches in communication with the exterior.
No sooner has a cleft been formed than its anterior border (i.e.the border nearer the head) becomes raised into a thick lip or fold, thevisceralorbranchial fold. Each cleft has its own fold on its anterior border, and in addition the posterior border of the fourth or last visceral cleft is raised into a similar fold. There are thusfivevisceral folds tofourvisceral clefts (figs.112and113). The last two folds however, and especially the last, are not nearly so thick and prominent as the other three, the second being the broadest and most conspicuous of all. The first fold meets, or nearly meets, its fellow in the middle line in front, but the second falls short of reaching the middle line, and the third, fourth and fifth do so in an increasing degree. Thus in front views of the neck a triangular space with its apex directed towards the head is observed between the ends of the several folds (fig. 113A).
Into this space the pleuroperitoneal cavity extends, the somatopleure separating from the splanchnopleure along the ends of the folds; and it is here that the aorta plunges into the mesoblast of the body.
The history of these most important visceral folds and clefts will be dealt with in detail hereafter; meanwhile I may say that in the Chick and higher Vertebrates the first three pairs of folds are those which call for most notice.
The first fold on either side, increasing rapidly in size and prominence, does not, like the others, remain single, but sends off in the course of the third day a branch or bud-like process from its upper edge (fig. 113). This branch, starting from near the outer end of the fold, runs forwards and upwards in front of the stomodæum, tending to meet the corresponding branchfrom the fold on the other side, at a point in the middle line nearer the front of the head than the junction of the main folds (fig. 113,sm). The two branches do not quite meet, being separated by a median process, which at the same time grows down from the extreme front of the head, and against which they abut (fig. 120,k). Between the main folds, which are directed somewhat downwards and their branches which slant upwards, the somewhat lozenge-shaped stomodæum is placed, which, as the folds become more and more prominent, grows deeper and deeper (fig. 120A). The main folds form the mandibular arch, and their branches the maxillary processes, and the descending process which helps to complete the anterior margin of the stomodæum or oral cavity is called, from the parts which will be formed out of it, thefrontonasal process.
Illustration: Figure 115Fig. 115. Transverse section through the dorsal region of an Embryo Chick of 45 hours.M.c.medullary canal;P.v.mesoblastic somite;W.d.Wolffian duct;So.Somatopleure;S.p.Splanchnopleure;p.p.pleuroperitoneal cavity;ao.aorta;v.blood-vessels;w.germinal wall;ch.notochord;op.junction between area opaca and area pellucida.
Fig. 115. Transverse section through the dorsal region of an Embryo Chick of 45 hours.M.c.medullary canal;P.v.mesoblastic somite;W.d.Wolffian duct;So.Somatopleure;S.p.Splanchnopleure;p.p.pleuroperitoneal cavity;ao.aorta;v.blood-vessels;w.germinal wall;ch.notochord;op.junction between area opaca and area pellucida.
In two succeeding pairs of visceral folds, which correspond with the hyoid and first branchial arches of the Ichthyopsida, are developed the parts of the hyoid bone, which will be bestconsidered in connection with the development of the skull. The last two disappear in the Chick without giving rise to any permanent structures. The external opening of the first viscerali.e.hyomandibular cleft becomes closed[66], but the inner part of the cleft, opening into the mouth, gives rise to the Eustachian tube and the tympanic cavity, the latter being formed as a special diverticulum.
Part of the membranous mandibular and hyoid arches form a wall round the dorsal part of the original opening of this cleft, and so give rise to the meatus auditorius externus. At the bottom of this is placed the tympanic membrane, which is probably derived from the tissue which grows over the dorsal part of the opening of the first cleft. It is formed of an external epiblast epithelium, a middle layer of mesoblast, and an internal hypoblastic epithelium.
Illustration: Figure 116Fig. 116. Transverse section through the trunk of a Duck embryo with about twenty-four mesoblastic somites.am.amnion;so.somatopleure;sp.splanchnopleure;wd.Wolffian duct;st.segmental tube;ca.v.cardinal vein;ms.muscle-plate;sp.g.spinal ganglion;sp.c.spinal cord;ch.notochord;ao.aorta;hy.hypoblast.
Fig. 116. Transverse section through the trunk of a Duck embryo with about twenty-four mesoblastic somites.am.amnion;so.somatopleure;sp.splanchnopleure;wd.Wolffian duct;st.segmental tube;ca.v.cardinal vein;ms.muscle-plate;sp.g.spinal ganglion;sp.c.spinal cord;ch.notochord;ao.aorta;hy.hypoblast.
The general nature of the changes, which take place in the trunk between the commencement of the second half of the second day and the end of the third day, is illustrated by the sectionsfigs.115,116,117.
Illustration: Figure 117Fig. 117. Section through the dorsal region of an embryo Chick at the end of the third day.Am.amnion;m.p.muscle-plate.C.V.cardinal vein.Ao.dorsal aorta. The section passes through the point where the dorsal aorta is just commencing to divide into two branches.Ch.notochord;W.d.Wolffian duct;W.b.commencing differentiation of the mesoblast cells to form the Wolffian body;ep.epiblast;So.somatopleure;Sp.splanchnopleure;hy.hypoblast. The section passes through the point where the digestive canal communicates with the yolk-sack, and is consequently stillopenbelow.
Fig. 117. Section through the dorsal region of an embryo Chick at the end of the third day.Am.amnion;m.p.muscle-plate.C.V.cardinal vein.Ao.dorsal aorta. The section passes through the point where the dorsal aorta is just commencing to divide into two branches.Ch.notochord;W.d.Wolffian duct;W.b.commencing differentiation of the mesoblast cells to form the Wolffian body;ep.epiblast;So.somatopleure;Sp.splanchnopleure;hy.hypoblast. The section passes through the point where the digestive canal communicates with the yolk-sack, and is consequently stillopenbelow.
In the earliest of these sections there is not a trace of a folding off of the embryo from the yolk, and the body walls are quite horizontal. In the second section (fig. 116), from an embryo of about two days, the body walls are already partially inclined, and the splanchnopleure is very distinctly folded inwards. There is a considerable space between the notochord and the hypoblast, which forms the rudiment of the mesentery.
In the third section (fig. 117) the body walls have become nearly vertical, the folding of the splanchnopleure is nearly completed, and it is only for a small region that the alimentary tract is open, by the vitelline duct, to the yolk-sack.
Illustration: Figure 118Fig. 118. Embryo Chick at the end of the fourth day seen as a transparent object.The amnion has been completely removed, the cut end of the somatic stalk is shewn atS.S.with the allantois (Al) protruding from it.C.H.cerebral hemisphere;F.B.vesicle of the third ventricle with the pineal gland (Pn) projecting from its summit;M.B.mid-brain;Cb.cerebellum.IV. V.fourth ventricle;L.lens;ch.s.choroid slit. Owing to the growth of the optic cup the two layers of which it is composed cannot any longer be seen from the surface, but the retinal surface of the layer alone is visible.Cen.V.auditory vesicle;s.m.superior maxillary process;1 F,2 F, etc. first, second, third and fourth visceral arches;V.fifth nerve sending one branch to the eye, the ophthalmic branch, and another to the first visceral arch;VII.seventh nerve passing to the second visceral arch;G.Ph.glossopharyngeal nerve passing towards the third visceral arch;Pg.pneumogastric nerve passing towards the fourth visceral arch;iv.investing mass. No attempt has been made in the figure to indicate the position of the dorsal wall of the throat, which cannot be easily made out in the living embryo;ch.notochord. The front end of this cannot be seen in the living embryo. It does not end however as shewn in the figure, but takes a sudden bend downwards and then terminates in a point.Ht.heart seen through the walls of the chest;M.P.muscle-plates.W.wing;H.L.hind limb. Beneath the hind limb is seen the curved tail.
Fig. 118. Embryo Chick at the end of the fourth day seen as a transparent object.The amnion has been completely removed, the cut end of the somatic stalk is shewn atS.S.with the allantois (Al) protruding from it.C.H.cerebral hemisphere;F.B.vesicle of the third ventricle with the pineal gland (Pn) projecting from its summit;M.B.mid-brain;Cb.cerebellum.IV. V.fourth ventricle;L.lens;ch.s.choroid slit. Owing to the growth of the optic cup the two layers of which it is composed cannot any longer be seen from the surface, but the retinal surface of the layer alone is visible.Cen.V.auditory vesicle;s.m.superior maxillary process;1 F,2 F, etc. first, second, third and fourth visceral arches;V.fifth nerve sending one branch to the eye, the ophthalmic branch, and another to the first visceral arch;VII.seventh nerve passing to the second visceral arch;G.Ph.glossopharyngeal nerve passing towards the third visceral arch;Pg.pneumogastric nerve passing towards the fourth visceral arch;iv.investing mass. No attempt has been made in the figure to indicate the position of the dorsal wall of the throat, which cannot be easily made out in the living embryo;ch.notochord. The front end of this cannot be seen in the living embryo. It does not end however as shewn in the figure, but takes a sudden bend downwards and then terminates in a point.Ht.heart seen through the walls of the chest;M.P.muscle-plates.W.wing;H.L.hind limb. Beneath the hind limb is seen the curved tail.
These three sections further illustrate (1) the gradual differentiationof the mesoblastic somites (fig. 115,P.v) into (a) the muscle-plates (figs.116,msand117,m.p), and (b) the tissue to form the vertebral bodies and adjacent connective tissue; (2) the formation of a mass of tissue between the lateral plates and the mesoblastic somites (fig. 115), known as the intermediate cell mass, on the dorsal side of which the Wolffian duct is formed, while the intermediate cell mass itself breaks up into the segmental tubes (fig. 116,st) and connective tissue of the Wolffian body.
Illustration: Figure 119Fig. 119. Section through the lumbar region of an embryo Chick at the end of the fourth day.n.c.neural canal;p.r.posterior root of spinal nerve with ganglion;a.r.anterior root of spinal nerve;A.G.C.anterior grey column of spinal cord;A.W.C.anterior white column of spinal cord just commencing to be formed, and not very distinctly marked in the figure;m.p.muscle-plate;ch.notochord;W.R.Wolffian ridge;AO.dorsal aorta;V.c.a.posterior cardinal vein;W.d.Wolffian duct;W.b.Wolffian body, consisting of tubules and Malpighian bodies;g.e.germinal epithelium;d.alimentary canal;M.commencing mesentery;SO.somatopleure;SP.splanchnopleure;V.blood-vessels;pp.pleuroperitoneal cavity.
Fig. 119. Section through the lumbar region of an embryo Chick at the end of the fourth day.n.c.neural canal;p.r.posterior root of spinal nerve with ganglion;a.r.anterior root of spinal nerve;A.G.C.anterior grey column of spinal cord;A.W.C.anterior white column of spinal cord just commencing to be formed, and not very distinctly marked in the figure;m.p.muscle-plate;ch.notochord;W.R.Wolffian ridge;AO.dorsal aorta;V.c.a.posterior cardinal vein;W.d.Wolffian duct;W.b.Wolffian body, consisting of tubules and Malpighian bodies;g.e.germinal epithelium;d.alimentary canal;M.commencing mesentery;SO.somatopleure;SP.splanchnopleure;V.blood-vessels;pp.pleuroperitoneal cavity.
Various other features in the development of the vascular system, general mesoblast, etc., are also represented in these sections. It may more especially be noted that there are at first two widely separated dorsal aortæ, which gradually approach (figs.115and116); and meeting first of all in front finally coalesce (figs.117and119) for their whole length.
The general appearance of the embryo of the fourth day may be gathered fromfig. 118.
Illustration: Figure 120Fig. 120. Head of a Chick from below on the sixth and seventh days of incubation.(From Huxley.)Ia. cerebral vesicles;a.eye, in which the remains of the choroid slit can still be seen in A;g.nasal pits;k.frontonasal process;l.superior maxillary process;1.inferior maxillary process or first visceral arch;2.second visceral arch;x.first visceral cleft.In A the cavity of the mouth is seen enclosed by the frontonasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over and converted into canals.In B the external opening of the mouth has become much constricted, but it is still enclosed by the frontonasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below.The superior maxillary processes have united with the frontonasal process, along nearly the whole length of the latter.
Fig. 120. Head of a Chick from below on the sixth and seventh days of incubation.(From Huxley.)Ia. cerebral vesicles;a.eye, in which the remains of the choroid slit can still be seen in A;g.nasal pits;k.frontonasal process;l.superior maxillary process;1.inferior maxillary process or first visceral arch;2.second visceral arch;x.first visceral cleft.In A the cavity of the mouth is seen enclosed by the frontonasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over and converted into canals.In B the external opening of the mouth has become much constricted, but it is still enclosed by the frontonasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below.The superior maxillary processes have united with the frontonasal process, along nearly the whole length of the latter.
The changes which have taken place consist for the most part in the further development of the parts already present, and do not need to be specified in detail. The most important event of the day is perhaps the formation of the limbs. They appear as outgrowths from a slightly marked lateral ridge (fig. 119,WR), which runs on the level of the lower end of the muscle-plates fornearly the whole length of the trunk. This ridge is known as the Wolffian ridge. The first trace of the limbs can be seen towards the end of the third day; and their appearance at the end of the fourth day is shown infig. 118,WandHL.
A section through the trunk of the embryo on the fourth day is represented infig. 119. The section passes through the region of the trunk behind the vitelline duct. The mesentery (M) is very much deeper and thinner than on the previous day. The notochord has become invested by a condensed mesoblastic tissue, which will give rise to the vertebral column. The two dorsal aortæ have now completely coalesced into the single dorsal aorta, and the Wolffian body has reached a far more complete development.
In the course of the fifth day the face begins to assume a less embryonic character, and by the sixth and succeeding days presents distinctive avian characters.
The general changes which take place between the sixth day and the time of hatching do not require to be specified in detail.
Fœtal Membranes.
The Reptilia, Aves and Mammalia are distinguished from the Ichthyopsida by the possession of certain provisional fœtal membranes, known as theamnionandallantois.
As the mode of development of these membranes may be most conveniently studied in the Chick, I have selected this type for their detailed description.
The Amnion. The amnion is a peculiar sack which envelopes and protects the embryo.
At the end of the first day of incubation, when the cleavage of the mesoblast has somewhat advanced, there appears, a little way in front of the semilunar head-fold, a second fold (fig. 102, alsofig. 121C,afandfig. 122,Am), running more or less parallel or rather concentric with the first and not unlike it in general appearance, though differing widely from it in nature. This second fold gives rise to the amnion, and is limited entirely to the somatopleure. Rising up as a semilunar fold with its concavity directed towards the embryo (fig. 121C,af), as it increases in height itis gradually drawn backwards over the developing head of the embryo. The fold thus covering the head is in due time accompanied by similar folds of somatopleure, starting at some little distance behind the tail, and at some little distance from the side (fig. 121 C, D,E, F, and116,am). In this way theembryo becomes surrounded by a series of folds of thin somatopleure, which form a continuous wall all round it. All are drawn gradually over the body of the embryo, and at last meet and completely coalesce (fig. 121, H, I, and117,Am), all traces of their junction being removed. Beneath these united folds there is therefore a cavity, within which the embryo lies (fig. 121H,ae). This cavity is the cavity of the amnion.
Illustration: Figure 121Fig. 121.A to N forms a series of purely diagrammatic representations introduced to facilitate the comprehension of the manner in which the body of the embryo is formed, and of the various relations of the yolk-sack, amnion, and allantois.In allvtis the vitelline membrane, placed, for convenience sake, at some distance from its contents, and represented as persisting in the later stages; in reality it is in direct contact with the blastoderm or yolk, and early ceases to have a separate existence. In alleindicates the embryo proper;ppthe general pleuroperitoneal space with its extension between the membranes;afthe folds of the amnion;athe amnion proper;aeoracthe cavity holding the liquor amnii;althe allantois;a´the alimentary canal;yorysthe yolk or yolk-sack.A, which may be considered as a vertical section taken longitudinally along the axis of the embryo, represents the relations of the parts of the egg at the time of the first appearance of the head-fold, seen on the right-hand side of the embryoe. The blastoderm is spreading both behind (to the left hand in the figure), and in front (to right hand) of the head-fold, its limits being indicated by the shading and thickening for a certain distance of the margin of the yolky. As yet there is no fold on the left side ofecorresponding to the head-fold on the right.B is a vertical transverse section of the same period drawn for convenience sake on a larger scale (it should have been made flatter and less curved). It shews that the blastoderm (vertically shaded) is extending laterally as well as fore and aft, in fact in all directions; but there are no lateral folds, and therefore no lateral limits to the body of the embryo as distinguished from the blastoderm.Incidentally it shews the formation of the medullary groove by the rising up of the laminæ dorsales. Beneath the section of the groove is seen the rudiment of the notochord. On either side a line indicates the cleavage of the mesoblast just commencing.In C, which represents a vertical longitudinal section of later date, both head-fold (on the right) and tail-fold (on the left) have advanced considerably. The alimentary canal is therefore closed in, both in front and behind, but is in the middle still widely open to the yolkybelow. Though the axial parts of the embryo have become thickened by growth, the body-walls are still thin; in them however is seen the cleavage of the mesoblast, and the divergence of the somatopleure and splanchnopleure. The splanchnopleure both at the head and at the tail is folded in to a greater extent than the somatopleure, and forms the still wide splanchnic stalk. At the end of the stalk, which is as yet short, it bends outwards again and spreads over the surface of the yolk. The somatopleure, folded in less than the splanchnopleure to form the wider somatic stalk, sooner bends round and runs outwards again. At a little distance from both the head and the tail it is raised up into a fold,af,af, that in front of the head being the highest. These are the amniotic folds. Descending from either fold,it speedily joins the splanchnopleure again, and the two, once more united into an uncleft membrane, extend some way downwards over the yolk, the limit or outer margin of the opaque area not being shewn. All the space between the somatopleure and the splanchnopleure is shaded with dots,pp. Close to the body this space may be called the pleuroperitoneal cavity; but outside the body it runs up into either amniotic fold, and also extends some little way over the yolk.D represents the tail end at about the same stage on a more enlarged scale, in order to illustrate the position of the allantoisal(which was for the sake of simplicity omitted in C), shewn as a bud from the splanchnopleure, stretching downwards into the pleuroperitoneal cavitypp. The dotted area representing as before the whole space between the splanchnopleure and the somatopleure, it is evident that a way is open for the allantois to extend from its present position into the space between the two limbs of the amniotic foldaf.Illustration: Figure 121aE, also a longitudinal section, represents a stage still farther advanced. Both splanchnic and somatic stalks are much narrowed, especially the former, the cavity of the alimentary canal being now connected with the cavity of the yolk by a mere canal. The folds of the amnion are spreading over the top of the embryo and nearly meet. Each fold consists of two walls or limbs, the space between which (dotted) is as before merely a part of the space between the somatopleure and splanchnopleure. Between these arched amniotic folds and the body of the embryo is a space not as yet entirely closed in.F represents on a different scale a transverse section of E taken through the middle of the splanchnic stalk. The dark ring in the body of the embryo shews the position of the neural canal, below which is a black spot, marking the notochord. On either side of the notochord the divergence of somatopleure and splanchnopleure is obvious. The splanchnopleure, more or less thickened, is somewhat bent in towards the middle line, but the two sides do not unite, the alimentary canal being as yet open below at this spot; after converging somewhat they diverge again and run outwards over the yolk. The somatopleure, folded in to some extent to form the body-walls, soon bends outwards again, and is almost immediately raised up into the lateral folds of the amnionaf. The continuity of the pleuroperitoneal cavity, within the body, with the interior of the amniotic fold, outside the body, is evident; both cavities are dotted.G, which corresponds to D at a later stage, is introduced to shew the manner in which the allantois, now a considerable hollow body, whose cavity is continuous with that of the alimentary canal, becomes directed towards the amniotic fold.In H a longitudinal, and I a transverse section of later date, great changes have taken place. The several folds of the amnion have met and coalesced above the body of the embryo. The inner limbs of the several folds have united into a single membrane (a), which encloses a space (aeorac) round the embryo. This membraneais the amnion proper, and the cavity within it,i.e.between it and the embryo, is the cavity of the amnion containing the liquor amnii. The allantois is omitted for the sake of simplicity.It will be seen that the amnionanow forms in every direction the termination of the somatopleure; the peripheral portions of the somatopleure, the united outer or descending limbs of the foldsafin C, D, F, G having been cut adrift, and now forming an independent continuous membrane, the serous membrane, immediately underneath the vitelline membrane.In I the splanchnopleure is seen converging to complete the closure of the alimentary canala´even at the stalk (elsewhere the canal has of course long been closedin), and then spreading outwards as before over the yolk. The point at which it unites with the somatopleure, marking the extreme limit of the cleavage of the mesoblast, is now much nearer the lower pole of the diminished yolk.As a result of these several changes, a great increase in the dotted space has taken place. It is now possible to pass from the actual peritoneal cavity within the body, on the one hand round a great portion of the circumference of the yolk, and on the other hand above the amniona, in the space between it and the serous envelope.Into this space the allantois is seen spreading in K atal.
Fig. 121.
A to N forms a series of purely diagrammatic representations introduced to facilitate the comprehension of the manner in which the body of the embryo is formed, and of the various relations of the yolk-sack, amnion, and allantois.In allvtis the vitelline membrane, placed, for convenience sake, at some distance from its contents, and represented as persisting in the later stages; in reality it is in direct contact with the blastoderm or yolk, and early ceases to have a separate existence. In alleindicates the embryo proper;ppthe general pleuroperitoneal space with its extension between the membranes;afthe folds of the amnion;athe amnion proper;aeoracthe cavity holding the liquor amnii;althe allantois;a´the alimentary canal;yorysthe yolk or yolk-sack.A, which may be considered as a vertical section taken longitudinally along the axis of the embryo, represents the relations of the parts of the egg at the time of the first appearance of the head-fold, seen on the right-hand side of the embryoe. The blastoderm is spreading both behind (to the left hand in the figure), and in front (to right hand) of the head-fold, its limits being indicated by the shading and thickening for a certain distance of the margin of the yolky. As yet there is no fold on the left side ofecorresponding to the head-fold on the right.B is a vertical transverse section of the same period drawn for convenience sake on a larger scale (it should have been made flatter and less curved). It shews that the blastoderm (vertically shaded) is extending laterally as well as fore and aft, in fact in all directions; but there are no lateral folds, and therefore no lateral limits to the body of the embryo as distinguished from the blastoderm.Incidentally it shews the formation of the medullary groove by the rising up of the laminæ dorsales. Beneath the section of the groove is seen the rudiment of the notochord. On either side a line indicates the cleavage of the mesoblast just commencing.In C, which represents a vertical longitudinal section of later date, both head-fold (on the right) and tail-fold (on the left) have advanced considerably. The alimentary canal is therefore closed in, both in front and behind, but is in the middle still widely open to the yolkybelow. Though the axial parts of the embryo have become thickened by growth, the body-walls are still thin; in them however is seen the cleavage of the mesoblast, and the divergence of the somatopleure and splanchnopleure. The splanchnopleure both at the head and at the tail is folded in to a greater extent than the somatopleure, and forms the still wide splanchnic stalk. At the end of the stalk, which is as yet short, it bends outwards again and spreads over the surface of the yolk. The somatopleure, folded in less than the splanchnopleure to form the wider somatic stalk, sooner bends round and runs outwards again. At a little distance from both the head and the tail it is raised up into a fold,af,af, that in front of the head being the highest. These are the amniotic folds. Descending from either fold,it speedily joins the splanchnopleure again, and the two, once more united into an uncleft membrane, extend some way downwards over the yolk, the limit or outer margin of the opaque area not being shewn. All the space between the somatopleure and the splanchnopleure is shaded with dots,pp. Close to the body this space may be called the pleuroperitoneal cavity; but outside the body it runs up into either amniotic fold, and also extends some little way over the yolk.D represents the tail end at about the same stage on a more enlarged scale, in order to illustrate the position of the allantoisal(which was for the sake of simplicity omitted in C), shewn as a bud from the splanchnopleure, stretching downwards into the pleuroperitoneal cavitypp. The dotted area representing as before the whole space between the splanchnopleure and the somatopleure, it is evident that a way is open for the allantois to extend from its present position into the space between the two limbs of the amniotic foldaf.
Illustration: Figure 121a
E, also a longitudinal section, represents a stage still farther advanced. Both splanchnic and somatic stalks are much narrowed, especially the former, the cavity of the alimentary canal being now connected with the cavity of the yolk by a mere canal. The folds of the amnion are spreading over the top of the embryo and nearly meet. Each fold consists of two walls or limbs, the space between which (dotted) is as before merely a part of the space between the somatopleure and splanchnopleure. Between these arched amniotic folds and the body of the embryo is a space not as yet entirely closed in.F represents on a different scale a transverse section of E taken through the middle of the splanchnic stalk. The dark ring in the body of the embryo shews the position of the neural canal, below which is a black spot, marking the notochord. On either side of the notochord the divergence of somatopleure and splanchnopleure is obvious. The splanchnopleure, more or less thickened, is somewhat bent in towards the middle line, but the two sides do not unite, the alimentary canal being as yet open below at this spot; after converging somewhat they diverge again and run outwards over the yolk. The somatopleure, folded in to some extent to form the body-walls, soon bends outwards again, and is almost immediately raised up into the lateral folds of the amnionaf. The continuity of the pleuroperitoneal cavity, within the body, with the interior of the amniotic fold, outside the body, is evident; both cavities are dotted.G, which corresponds to D at a later stage, is introduced to shew the manner in which the allantois, now a considerable hollow body, whose cavity is continuous with that of the alimentary canal, becomes directed towards the amniotic fold.In H a longitudinal, and I a transverse section of later date, great changes have taken place. The several folds of the amnion have met and coalesced above the body of the embryo. The inner limbs of the several folds have united into a single membrane (a), which encloses a space (aeorac) round the embryo. This membraneais the amnion proper, and the cavity within it,i.e.between it and the embryo, is the cavity of the amnion containing the liquor amnii. The allantois is omitted for the sake of simplicity.It will be seen that the amnionanow forms in every direction the termination of the somatopleure; the peripheral portions of the somatopleure, the united outer or descending limbs of the foldsafin C, D, F, G having been cut adrift, and now forming an independent continuous membrane, the serous membrane, immediately underneath the vitelline membrane.In I the splanchnopleure is seen converging to complete the closure of the alimentary canala´even at the stalk (elsewhere the canal has of course long been closedin), and then spreading outwards as before over the yolk. The point at which it unites with the somatopleure, marking the extreme limit of the cleavage of the mesoblast, is now much nearer the lower pole of the diminished yolk.As a result of these several changes, a great increase in the dotted space has taken place. It is now possible to pass from the actual peritoneal cavity within the body, on the one hand round a great portion of the circumference of the yolk, and on the other hand above the amniona, in the space between it and the serous envelope.Into this space the allantois is seen spreading in K atal.