Illustration: Figure 128Fig. 128. Four transverse sections through the hinder end of a young embryo of Lacerta muralis.Sections A and B pass through the whole embryo, while C and D only pass through the allantois, which at this stage projects backwards into the section of the body cavity behind the primitive streak.ne.neurenteric canal;pr.primitive streak;hg.hindgut;hy.hypoblast;pp.body cavity;am.amnion;se.serous envelope (outer limb of the amnion fold not yet separated from the inner limb or true amnion);al.allantois;me.mesoblastic wall of the allantois;v.vessels passing to the allantois.
Fig. 128. Four transverse sections through the hinder end of a young embryo of Lacerta muralis.Sections A and B pass through the whole embryo, while C and D only pass through the allantois, which at this stage projects backwards into the section of the body cavity behind the primitive streak.ne.neurenteric canal;pr.primitive streak;hg.hindgut;hy.hypoblast;pp.body cavity;am.amnion;se.serous envelope (outer limb of the amnion fold not yet separated from the inner limb or true amnion);al.allantois;me.mesoblastic wall of the allantois;v.vessels passing to the allantois.
In Lacerta muralis the history appears to be somewhat different, and it is more especially to be noticed that in this species the hindgut does not become closed till considerably after the completion of the neural canal. In a stage shortly after that last described, the neurenteric passage becomes narrower. The next stage which I have observed is considerably later. The neural canal has become completely closed, and the flexure of the embryo has already made its appearance. There is still a well-developed, though somewhat slit-like, neurenteric passage, but from the analogy of birds, it is not impossible that it may have in the meantime closed upand opened again. It has, in any case, the same relations as in the previous stage.
It leads from the end of the medullary canal (at the point where its walls are continuous with the cells of the primitive streak) round the end of the notochord, which here becomes continuous with the medullary cord, and so through the hypoblast. The latter layer is still a flat sheet without any lateral infolding; but it gives rise, behind the neurenteric passage, to a blind posteriorly directed diverticulum, placed in the body cavity behind the embryo, and opening at the ventral face of the apparent hind end of the primitive streak. There is very little doubt that this diverticulum is the commencing allantois.
At a somewhat later stage the arrangement of these parts has undergone some changes. Their relations are shewn in the sections represented infig. 128.
The foremost section (A) passes through the alimentary opening of the neurenteric passage (ne). Above this opening the section passes through the primitive streak (pr) close to its junction with the walls of the medullary canal. The hypoblast is folded in laterally, but the gut is still open below. The amnion is completely established. In the next section figured (B), the fourth of my series, the gut is completely closed in; and the mesoblast has united laterally with the axial tissue of the primitive streak. Vessels to supply the allantois are shewn atv.
The three following sections are not figured, but they present the same features as B, except that the primitive streak gets rapidly smaller, and the lumen of the gut narrower. The section following (C) represents, I believe, only the stalk of the allantoic diverticulum. This diverticulum appears to be formed as usual of hypoblast (hy) enveloped by splanchnic mesoblast (me), and projects into the section of the body cavity present behind the embryo. Its position in the body cavity is the cause of its somewhat peculiar appearance in the figure. Had the whole section been represented the allantois would have been enclosed in a space between the serous membrane (se) and a layer of splanchnic mesoblast below which has also been omitted in fig. B[72]. It still points directly backwards, as it primitively does in the chick,videfig. 123A, and Gasser,No.127,Pl.V.figs.1 and 2. I do not understand the apparently double character of the lumen of the allantois. In the next section (not figured) the lumen of the allantoic stalk is larger, but still apparently double, while in the last section (D) the lumen is considerably enlarged and single. The neurenteric canal appears to close shortly after the stage last described, though its further history has not been followed in detail.
General development of the Embryo.
The formation of the embryo commences with the appearance of the medullary plate, the sides of which soon grow up to form the medullary folds. The medullary groove is developed anteriorly before any trace of it is visible behind. In a general way the closure of the groove takes place as in Birds, but the anterior part of the body is very early folded off, sinks into the yolk, and becomes covered over by the amnion as by a hood (figs.127and129). All this takes place before the closure of the medullary canal; and the changes of this part are quite concealed from view.
Illustration: Figure 129Fig. 129. Surface view of a young embryo of Lacerta muralis.am.amnion;pr.primitive streak.
Fig. 129. Surface view of a young embryo of Lacerta muralis.am.amnion;pr.primitive streak.
The closure of the medullary canal commences in the neck, and extends forwards and backwards; and the whole region of the brain becomes closed in, while the groove is still largely open behind.
The later stages in the development of the Lacertilian embryo do not require a detailed description, as they present the closest analogy with those already described for Aves. The embryo soon turns on to its left side; and then, becoming continuously folded off from the yolk, passes through the series of changes of form with which the reader is already familiar. An advanced embryo is represented infig. 130. The early development and great length of the tail, which is spirally coiled on the ventral surface, is a special feature to which the attention of the reader may be called.
Embryonic Membranes and Yolk-Sack.
The early development of the cephalic portion of the amnion has already been alluded to. The first traces of it become apparent while the medullary groove is still extremely shallow. The medullary plate in the region of the head forms an axial strip of a thickish plate of epiblast. The edge of this platecoincides with the line of the amniotic fold, and as this fold rises up the two sides of the plate become bent over the embryo and give rise to the inner limb of the amnion or amnion proper. The section (fig. 127), representing the origin of the amniotic hood of the head, shews very well how the space between the two limbs of the amnion is continuous with the body cavity. The amnion very early completely encloses the embryo (fig. 128A and B), and its external limb or serous membrane, after separating from the true amnion, soon approaches and fuses with the vitelline membrane.
Illustration: Figure 130Fig. 130. Advanced embryo of Lacerta muralis as an opaque object[73].The embryo was 7mm.in length in the curled up state.fb.fore-brain;mb.mid-brain;cb.cerebellum;au.auditory vesicle (closed);ol.olfactory pit;md.mandible;hy.hyoid arch;br.branchial arches;fl.fore-limb;hl.hind-limb.
Fig. 130. Advanced embryo of Lacerta muralis as an opaque object[73].The embryo was 7mm.in length in the curled up state.fb.fore-brain;mb.mid-brain;cb.cerebellum;au.auditory vesicle (closed);ol.olfactory pit;md.mandible;hy.hyoid arch;br.branchial arches;fl.fore-limb;hl.hind-limb.
The first development of the allantois as a diverticulum of the hypoblast covered by splanchnic mesoblast, at the apparent posterior end of the primitive streak, has been described onp.207. The allantois continues for some time to point directly backwards; but gradually assumes a more ventral direction; and, as it increases in size, extends into the space between the serous membrane and amnion, eventually to form a large, highly vascular, flattened sack immediately below the serous membrane.
The Yolk-Sack. The blastoderm spreads in the Lizard with very great rapidity over the yolk to form the yolk-sack. The early appearance of the area pellucida, or as it has been called by Kupffer and Benecke the embryonic shield, has already been noted. Outside this a vascular area, which has the same function asin the chick, is not long in making its appearance. In all Reptilia the vascular channels which arise in the vascular area, and the vessels carrying the blood to and from the vascular area, are very similar to those in the chick. In the Snake the sinus terminalis never attains so conspicuous a development and in Chelonia the stage with a pair of vitelline arteries is preceded by a stage in which the vascular area is supplied, as it permanently is in many Mammals, by numerous transverse arterial trunks, coming off from the dorsal aorta (Agassiz,No.164). The vascular area gradually envelops the whole yolk, although it does so considerably more slowly than the general blastoderm.
Ophidia. There is, as might have been anticipated, a very close correspondence in general development between the Lacertilia and Ophidia. The embryos of all the Amniota are, during part of their development, more or less spirally coiled about their long axis. This is well marked in the chick of the third day; it is still more pronounced in the Lizard (fig. 130); but it reaches its maximum in the Snake. The whole Snake embryo has at the time when most coiled (Dutrochet, Rathke) somewhat the form of a Trochus. The base of the spiral is formed by the head, while the majority of the coils are supplied by the tail. There are in all at this stage seven coils, and the spiral is right-handed.
Another point, which deserves notice in the Snake, is the absence in the embryo of all external trace of the limbs. It might have been anticipated, on the analogy of the branchial arches, that rudiments of the limbs would be preserved in the embryo even when limbs were absent in the adult. Such, however, is not the case. It is however very possible that rudiments of the branchial arches and clefts have been preserved because these structures were functional in the larva (Amphibia) after they ceased to have any importance in the adult; and that the limbs have disappeared even in the embryo because in the course of their gradual atrophy there was no advantage to the organism in their being specially preserved at any period of life[74].
Illustration: Figure 131Fig. 131. Chelone midas, first stage.Au.auditory capsule;br.1and2, branchial arches;C.carapace;E.eye;f.b.fore-brain;f.l.fore-limb;H.heart;h.b.hind-brain;h.l.hind-limb;hy.hyoid;m.b.mid-brain;mn.mandible;mx.p.maxillopalatine;N.nostril;u.umbilicus.
Fig. 131. Chelone midas, first stage.Au.auditory capsule;br.1and2, branchial arches;C.carapace;E.eye;f.b.fore-brain;f.l.fore-limb;H.heart;h.b.hind-brain;h.l.hind-limb;hy.hyoid;m.b.mid-brain;mn.mandible;mx.p.maxillopalatine;N.nostril;u.umbilicus.
Illustration: Figure 132Fig. 132. Chelone midas, second stage.Letters as in fig. 131.
Fig. 132. Chelone midas, second stage.Letters as in fig. 131.
Chelonia[75]. In their early development the Chelonia resemble,so far as is known, the Lacertilia. The amnion arises early, and soon forms a great cephalic hood. Before development has proceeded very far the embryo turns over on to its left side. The tail in many species attains a very considerabledevelopment (fig. 133). The chief peculiarity in the form of the embryo (figs.131,132, and133) is caused by the development of the carapace. The first rudiment of the carapace appears in the form of two longitudinal folds, extending above the line of insertion of the fore- and hind-limbs, which have already made their appearance (fig. 131). These folds are subsequently prolonged so as to mark out the area of the carapace on the dorsal surface. On the surface of this area there are formed the horny plates (tortoise shell), and in the mesoblast below the bony elements of the carapace (figs.132and133).
Illustration: Figure 133Fig. 133. Chelone midas, third stage.Letters as in fig. 131.r.rostrum.
Fig. 133. Chelone midas, third stage.Letters as in fig. 131.r.rostrum.
Immediately after hatching the yolk-sack becomes withdrawn into the body; while the external part of the allantois shrivels up.
Bibliography.
General.
(154)C. KupfferandBenecke.Die erste Entwicklung am Ei d. Reptilien.Königsberg, 1878.(155)C. Kupffer. “Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere.”Zoologischer Anzeiger,Vol.II. 1879,pp.520, 593, 612.
Lacertilia.
(156)F. M. Balfour. “On the early Development of the Lacertilia, together with some observations, etc.”Quart. J. of Micr. Science,Vol.XIX.1879.(157)Emmert u. Hochstetter. “Untersuchung üb. d. Entwick. d. Eidechsen in ihren Eiern.” Reil’sArchiv,Vol.X. 1811.(158)M. Lereboullet. “Développement de la Truite, du Lézard et du Limnée. II. Embryologie du Lézard.”An. Sci. Nat.,Ser.IV., Vol.XXVII. 1862.(159)W. K. Parker. “Structure and Devel. of the Skull in Lacertilia.”Phil. Trans., Vol. 170,p.2. 1879.(160)H. Strahl. “Ueb. d. Canalis myeloentericus d. Eidechse.”Schrift. d. Gesell. z. Beför. d. gesam. Naturwiss.Marburg.July23, 1880.
Ophidia.
(161)H. Dutrochet. “Recherches s. l. enveloppes du fœtus.”Phil. Trans., Paris,Vol.VIII. 1816.(162)W. K. Parker. “On the skull of the common Snake.”Phil. Trans.,Vol.169, PartII.1878.(163)H. Rathke.Entwick. d. Natter.Königsberg, 1839.
Chelonia.
(164)"L. Agassiz.Contributions to the Natural History of the United States,Vol.II. 1857.Embryology of the Turtle.(165)W. K. Parker. “On the development of the skull and nerves in the green Turtle.”Proc. of the Roy. Soc.,Vol.XXVIII. 1879.VidealsoNature, April 14, 1879, andChallenger Reports,Vol.I. 1880.(166)H. Rathke.Ueb. d. Entwicklung d. Schildkröten.Braunschweig, 1848.
Crocodilia.
(167)H. Rathke.Ueber die Entwicklung d. Krokodile.Braunschweig, 1866.
[71]Kupffer and Benecke (No.154) give a very different account from the above of the early Lacertilian development, more especially in what concerns the so-called neurenteric passage. They believe this structure to be closed below, and to form therefore a blind sack open externally. The open end of this sack they regard as the blastopore—an interpretation which accords with my own, but they regard the sack as the rudiment of the allantois, and hold that it is equivalent to the invaginated archenteron of Amphioxus. I need scarcely say that I believe Kupffer and Benecke to have made a mistake in denying the existence of the ventral opening of this organ. Kupffer in a subsequent paper (No.155) states that my descriptions of the structure of this organ do not correspond with the fact. I have perfect confidence in leaving the decision of this point to future observers, and may say that my observations have already been fully confirmed by Strahl (No.160), who has also added some observations on the later stages to which I shall hereafter have occasion to allude.[72]Owing to the difficulty of procuring material I have only been able to prepare the two sets of sections just described, and in the absence of a fuller series there are some points in the interpretation of the sections which must remain doubtful.[73]This figure was drawn for me by Professor Haddon.[74]It is very probable that in those Ophidia in which traces of limbs are still preserved, that more conspicuous traces would be found in the embryos than in the adults.[75]VideAgassiz (No.164), Kupffer and Benecke (No.154), and Parker (No.165).
[71]Kupffer and Benecke (No.154) give a very different account from the above of the early Lacertilian development, more especially in what concerns the so-called neurenteric passage. They believe this structure to be closed below, and to form therefore a blind sack open externally. The open end of this sack they regard as the blastopore—an interpretation which accords with my own, but they regard the sack as the rudiment of the allantois, and hold that it is equivalent to the invaginated archenteron of Amphioxus. I need scarcely say that I believe Kupffer and Benecke to have made a mistake in denying the existence of the ventral opening of this organ. Kupffer in a subsequent paper (No.155) states that my descriptions of the structure of this organ do not correspond with the fact. I have perfect confidence in leaving the decision of this point to future observers, and may say that my observations have already been fully confirmed by Strahl (No.160), who has also added some observations on the later stages to which I shall hereafter have occasion to allude.
[72]Owing to the difficulty of procuring material I have only been able to prepare the two sets of sections just described, and in the absence of a fuller series there are some points in the interpretation of the sections which must remain doubtful.
[73]This figure was drawn for me by Professor Haddon.
[74]It is very probable that in those Ophidia in which traces of limbs are still preserved, that more conspicuous traces would be found in the embryos than in the adults.
[75]VideAgassiz (No.164), Kupffer and Benecke (No.154), and Parker (No.165).
The classical researches of Bischoff on the embryology of several mammalian types, as well as those of other observers, have made us acquainted with the general form of the embryos of the Placentalia, and have shewn that, except in the earliest stages of development, there is a close agreement between them. More recently Hensen, Schäfer, Kölliker, Van Beneden and Lieberkühn have shed a large amount of light on the obscurer points of the earliest developmental periods, especially in the rabbit. For the early stages the rabbit necessarily serves as type; but there are grounds for thinking that not inconsiderable variations are likely to be met with in other species, and it is not at present easy to assign to some of the developmental features their true value. We have no knowledge of the early development of the Ornithodelphia or Marsupialia.
The ovum on leaving the ovary is received by the fimbriated extremity of the Fallopian tube, down which it slowly travels. It is still invested by the zona radiata, and in the rabbit an albuminous envelope is formed around it in its passage downwards. Impregnation takes place in the upper part of the Fallopian tube, and is shortly followed by the segmentation, which is remarkable amongst the Amniota for being complete.
Although this process (the details of which have been made known by the brilliant researches of Ed. van Beneden) has already been shortly dealt with as it occurs in the rabbit (Vol.II. p.98) it will be convenient to describe it again with somewhat greater detail.
The ovum first divides into two nearly equal spheres, of which one is slightly larger and more transparent than theother. The larger sphere and its products will be spoken of as the epiblastic spheres, and the smaller one and its products as the hypoblastic spheres, in accordance with their different destinations.
Both the spheres are soon divided into two, and each of the four so formed into two again; and thus a stage with eight spheres ensues. At the moment of their first separation these spheres are spherical, and arranged in two layers, one of them formed of the four epiblastic spheres, and the other of the four hypoblastic. This position is not long retained, but one of the hypoblastic spheres passes to the centre; and the whole ovum again takes a spherical form.
In the next phase of segmentation each of the four epiblastic spheres divides into two, and the ovum thus becomes constituted of twelve spheres, eight epiblastic and four hypoblastic. The epiblastic spheres have now become markedly smaller than the hypoblastic.
The four hypoblastic spheres next divide, giving rise, together with the eight epiblastic spheres, to sixteen spheres in all; which are nearly uniform in size. Of the eight hypoblastic spheres four soon pass to the centre, while the eight superficial epiblastic spheres form a kind of cup partially enclosing the hypoblastic spheres. The epiblastic spheres now divide in their turn, giving rise to sixteen spheres which largely enclose the hypoblastic spheres. The segmentation of both epiblastic and hypoblastic spheres continues, and in the course of it the epiblastic spheres spread further and further over the hypoblastic, so that at the close of segmentation the hypoblastic spheres constitute a central solid mass almost entirely surrounded by the epiblastic spheres. In a small circular area however the hypoblastic spheres remain for some time exposed at the surface (fig. 134A).
The whole process of segmentation is completed in the rabbit about seventy hours after impregnation. At its close the epiblast cells, as they may now be called, are clear, and have an irregularly cubical form; while the hypoblast cells are polygonal and granular, and somewhat larger than the epiblast cells.
The opening in the epiblastic layer where the hypoblast cells are exposed on the surface may for convenience be called withVan Beneden the blastopore, though it is highly improbable that it in any way corresponds with the blastopore of other vertebrate ova[76].
Illustration: Figure 134Fig. 134. Optical sections of a Rabbit’s ovum at two stages closely following upon the segmentation.(After E. van Beneden.)ep.epiblast;hy.primary hypoblast;bp.Van Beneden’s blastopore. The shading of the epiblast and hypoblast is diagrammatic.
Fig. 134. Optical sections of a Rabbit’s ovum at two stages closely following upon the segmentation.(After E. van Beneden.)ep.epiblast;hy.primary hypoblast;bp.Van Beneden’s blastopore. The shading of the epiblast and hypoblast is diagrammatic.
After its segmentation the ovum passes into the uterus. The epiblast cells soon grow over the blastopore and thus form a complete superficial layer. A series of changes next take place which result in the formation of what has been called theblastodermic vesicle. To Ed. van Beneden we owe the fullest account of these changes; to Hensen and Kölliker however we are also indebted for valuable observations, especially on the later stages in the development of this vesicle.
The succeeding changes commence with the appearance of a narrow cavity between the epiblast and hypoblast, which extends so as completely to separate these two layers except in the region adjoining the original site of the blastopore (fig. 134B)[77]. The cavity so formed rapidly enlarges, and with it the ovum also; which soon takes the form of a thin-walled vesicle with a large central cavity. This vesicle is the blastodermicvesicle. The greater part of its walls are formed of a single row of flattened epiblast cells; while the hypoblast cells form a small lens-shaped mass attached to the inner side of the epiblast cells (fig. 135).
Illustration: Figure 135Fig. 135. Rabbit’s ovum between 70-90 hours after impregnation.(After E. van Beneden.)bv.cavity of blastodermic vesicle (yolk-sack);ep.epiblast;hy.primitive hypoblast;Zp.mucous envelope (zona pellucida).
Fig. 135. Rabbit’s ovum between 70-90 hours after impregnation.(After E. van Beneden.)bv.cavity of blastodermic vesicle (yolk-sack);ep.epiblast;hy.primitive hypoblast;Zp.mucous envelope (zona pellucida).
In the Vespertilionidæ Van Beneden and Julin have shewn that the ovum undergoes at the close of segmentation changes of a more or less similar nature to those in the rabbit; the blastopore would however appear to be wider, and to persist even after the cavity of the blastodermic vesicle has commenced to be developed.
Although by this stage, which occurs in the rabbit between seventy and ninety hours after impregnation, the blastodermic vesicle has by no means attained its greatest dimensions, it has nevertheless grown from about 0.09mm.—the size of the ovum at the close of segmentation—to about 0.28. It is enclosed by a membrane formed from the zona radiata and the mucous layer around it. The blastodermic vesicle continues to enlarge rapidly, and during the process the hypoblastic mass undergoes important changes. It spreads out on the inner side of the epiblast and at the same time loses its lens-like form and becomes flattened. The central part of it remains however thicker, and is constituted of two rows of cells, while the peripheral part, the outer boundary of which is irregular, is formed of an imperfect layer of amœboid cells which continually spread further and further within the epiblast. The central thickening of the hypoblast forms an opaque circular spot on the blastoderm, which constitutes the commencement of theembryonic area.
The history of the stages immediately following, from about the commencement of the fifth day to the seventh day, when a primitive streak makes its appearance, is imperfectly understood, and has been interpreted very differently by Van Beneden (No.171) on the one hand and by Kölliker (184), Rauber (187) and Lieberkühn (186) on the other. I have myself in conjunction with my pupil, Mr Heape, also conducted some investigations on these stages, which have unfortunately not as yet led me to a completely satisfactory reconciliation of the opposing views.
Van Beneden states that about five days after impregnation the hypoblast cells in the embryonic area become divided into two distinct strata, an upper stratum of small cells adjoining the epiblast and a lower stratum of flattened cells which form the true hypoblast. At the edge of the embryonic area the hypoblast is continuous with a peripheral ring of the amœboid cells of the earlier stage, which now form, except at the edge of the ring, a continuous layer of flattened cells in contact with the epiblast. During the sixth day the flattened epiblast cells are believed by Van Beneden to become columnar. The embryonic area gradually extends itself, and as it does so becomes oval. A central lighter portion next becomes apparent, which gradually spreads, till eventually the darker part of the embryonic area forms a crescent at the posterior part of the now somewhat pyriform embryonic area. The lighter part is formed of columnar epiblast and hypoblast only, while in the darker area a layer of the mesoblast, derived from the intermediate layer of the fifth day, is also found. In this darker area the primitive streak originates early on the seventh day.
Kölliker, following the lines originally laid down by Rauber, has arrived at very different results. He starts from the three-layered condition described by Van Beneden for the fifth day, but does not give any investigations of his own as to the origin of the middle layer. He holds the outer layer to be a provisional layer of protective cells, forming part of the wall of the original vesicle, the middle layer he regards as the true epiblast and the inner layer as the hypoblast.
During the sixth day he finds that the cells of the outer layer gradually cease to form a continuous layer and finally disappear; while the cells of the middle layer become columnar, and form the columnar epiblast present in the embryonic area at the end of the sixth day. The mesoblast first takes its origin in the region and on the formation of the primitive streak.
The investigations of Heape and myself do not extend to the first formation of the intermediate layer found on the fifth day. We find on the sixth day in germinal vesicles of about 2.2-2.5 millimetres in diameter with embryonic areas of about .8mm.that the embryonic area (fig. 136) is throughout composed of:(1) A layer of flattened hypoblast cells;(2) A somewhat irregular layer of more columnar elements, in some places only a single row deep and in other places two or more rows deep.(3) Flat elements on the surface, which do not, however, form a continuous layer, and are intimately attached to the columnar cells below.
Our results as to the structure of the blastoderm at this stage closely correspond therefore with those of Kölliker, but on one important point we have arrived at a different conclusion. Kölliker states that he has never found the flattened elements in the act of becoming columnar. We believe that we have in many instances been able to trace them in the act of undergoing this change, and have attempted to shew this in our figure.
Our next oldest embryonic areas were somewhat pyriform measuring about 1.19mm.in length and .85 in breadth. Of these we have several, some from a rabbit in which we also met with younger still nearly circular areas. All of them had a distinctly marked posterior opacity forming a commencing primitive streak, though decidedly less advanced than in the blastoderm represented infig. 140. In the younger specimens the epiblast in front of the primitive streak was formed of a single row of columnar cells (fig. 138A), no mesoblast was present and the hypoblast formed a layer of flattened cells. In the region immediately in front of the primitive streak, an irregular layer of mesoblast cells was interposed between the epiblast and hypoblast. In the anterior part of the primitive streak itself (fig. 138B) there was a layer of mesoblast with a considerable lateral extension, while in the median line there was a distinct mesoblastic proliferation of epiblast cells. In the posterior sections the lateral extension of the mesoblast was less, but the mesoblast cells formed a thicker cord in the axial line.
Owing to the unsatisfactory character of our data the following attempt to fill in the history of the fifth and sixth days must be regarded as tentative[78]. At the commencement of the fifth day the central thickening, of what has been called above the primitive hypoblast, becomes divided into two layers: the lower of these is continuous with the peripheral hypoblast and is formed of flattened cells, while the upper one is formed of small rounded elements. The superficial epiblast again is formed of flattened cells.
During the fifth day remarkable changes take place in the epiblast of the embryonic area. It is probable that its constituentcells increase in number and become one by one columnar; and that in the process they press against the layer of rounded elements below them, so that the two layers cease to be distinguishable, and thewholeembryonic area acquires in section the characters represented infig. 136[79]. Towards the end of the sixth day the embryonic area becomes oval, but the changes which next take place are not understood. In the front part of the area only two layers of cells are found, (1) an hypoblast, and (2) an epiblast of columnar cells probably derived from the flattened epiblast cells of the earlier stages. In the posterior part of the blastoderm a middle layer is present (Van Beneden) in addition to the two other layers; and this layer probably originates from the middle layer which extended throughout the area at the beginning of the fifth day, and then became fused with the epiblast. The middle layer does not give rise to the whole of the eventual mesoblast, but only to part of it. From its origin it may be called the hypoblastic mesoblast, and it is probably equivalent to the hypoblastic mesoblast already described in the chick (pp. 154 and 155). The stage just described has only been met with by Van Beneden[80].
Illustration: Figure 136Fig. 136. Section through the nearly circular embryonic area of a Rabbit’s ovum of six days, nine hours and .8mm.in diameter.The section shews the peculiar character of the upper layer with a certain number of superficial flattened cells; and represents about half the breadth of the area.
Fig. 136. Section through the nearly circular embryonic area of a Rabbit’s ovum of six days, nine hours and .8mm.in diameter.The section shews the peculiar character of the upper layer with a certain number of superficial flattened cells; and represents about half the breadth of the area.
A diagrammatic view of the whole blastodermic vesicle at about the beginning of the seventh day is given infig. 137. The embryonic area is represented in white. The linegein B shews the extension of the hypoblast round the inner side of the vesicle. The blastodermic vesicle is therefore formed of three areas, (1)the embryonic area with three layers: this area is placed where the blastopore was originally situated. (2) The ring around the embryonic area where the walls of the vesicle are formed of epiblast and hypoblast. (3) The area beyond this again where the vesicle is formed of epiblast only[81].
Illustration: Figure 137Fig. 137. Views of the blastodermic vesicle of a Rabbit on the seventh day without the zona.A. from above, B. from the side. (From Kölliker.)ag.embryonic area;ge.boundary of the hypoblast.
Fig. 137. Views of the blastodermic vesicle of a Rabbit on the seventh day without the zona.A. from above, B. from the side. (From Kölliker.)ag.embryonic area;ge.boundary of the hypoblast.
The changes which next take place begin with the formation of a primitive streak, homologous with, and in most respects similar to, the primitive streak in Birds. The formation of the streak is preceded by that of a clear spot near the middle of the blastoderm, forming the nodal point of Hensen. This spot subsequently constitutes the front end of the primitive streak.
The history of the primitive streak was first worked out in a satisfactory manner by Hensen (No.182), from whom however I differ in admitting the existence of a certain part of the mesoblast before its appearance.
Early on the seventh day the embryonic area becomes pyriform, and at its posterior and narrower end a primitive streak makes its appearance, which is due to a proliferation of rounded cells from the epiblast. At the time when this proliferationcommences the layer of hypoblastic mesoblast is present, especially just in front of, and at the sides of, the anterior part of the streak; but no mesoblast is found in the anterior part of the embryonic area. These features are shewn infig. 138A and B. The mesoblast derived from the proliferation of the epiblast soon joins the mesoblast already present; though in many sections itseems possible to trace a separation between the two parts (fig. 139B) of the mesoblast.
Illustration: Figure 138Fig. 138. Two sections through oval blastoderms of a Rabbit on the seventh day. The length of the area was about 1.2mm.and its breadth about .86mm.A. Through the region of the blastoderm in front of the primitive streak; B. through the front part of the primitive streak;ep.epiblast;m.mesoblast;hy.hypoblast;pr.primitive streak.
Fig. 138. Two sections through oval blastoderms of a Rabbit on the seventh day. The length of the area was about 1.2mm.and its breadth about .86mm.A. Through the region of the blastoderm in front of the primitive streak; B. through the front part of the primitive streak;ep.epiblast;m.mesoblast;hy.hypoblast;pr.primitive streak.
Illustration: Figure 139Fig. 139. Two transverse sections through the embryonic area of an embryo Rabbit of seven days.The embryo has nearly the structure represented in fig. 140.A. is taken through the anterior part of the embryonic area. It represents about half the breadth of the area, and there is no trace of a medullary groove or of the mesoblast.B. Is taken through the posterior part of the primitive streak.ep.epiblast;hy.hypoblast.
Fig. 139. Two transverse sections through the embryonic area of an embryo Rabbit of seven days.The embryo has nearly the structure represented in fig. 140.A. is taken through the anterior part of the embryonic area. It represents about half the breadth of the area, and there is no trace of a medullary groove or of the mesoblast.B. Is taken through the posterior part of the primitive streak.ep.epiblast;hy.hypoblast.
During the seventh day the primitive streak becomes a more pronounced structure, the mesoblast in its neighbourhood increases in quantity, while an axial groove—the primitive groove—is formed on its upper surface. The mesoblastic layer in front of the primitive streak becomes thicker, and, in the two-layered region in front, the epiblast becomes several rows deep (fig. 139A).