[5]The following classification of the Urochorda is adopted in the present chapter.I.Caducichordata.A. SimpliciaSolitariaex. Ascidia.Socialiaex. Clavellina.B. CompositaSedentariaex. Botryllus.Natantiaex. Pyrosoma.C. ConsertaSalpidæ.Doliolidæ.II.Perennichordata.Ex. Appendicularia.[6]It is more probable that this part of the alimentary tract is equivalent to the postanal gut of many Vertebrata, which is at first a complete tube, but disappears later by the simple absorption of the walls.[7]It is probable that these papillæ are very primitive organs of the Chordata. Structures, which are probably of the same nature, are formed behind the mouth in the larvæ of Amphibia, and in front of the mouth in the larvæ of Ganoids (Acipenser, Lepidosteus), and are used by these larvæ for attaching themselves.[8]For a fuller account of the organs of sensevidethe chapters on the eye and ear.[9]The account of the multiplication of the branchial clefts is taken from Krohn’s paper on Phallusia mammillata (No.24), but there is every reason to think that it holds true in the main for simple Ascidians.[10]In the asexually produced buds of Ascidians the atrial cavity appears, with the exception of the external opening, to be formed from the primitive branchial sack. In the buds of Pyrosoma however it arises independently. These peculiarities in the buds cannot weigh against the embryonic evidence that the atrial cavity arises from involutions of the epiblast, and they may perhaps be partially explained by the fact that in the formation of the visceral clefts outgrowths of the branchial sack meet the atrial involutions.[11]From Todaro’s latest paper (No.39) it would seem the segmentation cavity has very peculiar relations.[12]Brooks takes a very different view of the nature of the parts in Salpa. He says,No.7,p.322, “The atrium of Salpa, when first observed, was composed of two broad lateral atria within the body cavity, one on each side of the branchial sack, and a very small mid-atrium.... The lateral atria do not however, as in most Tunicata, remain connected with the mid-atrium, and unite with the wall of the branchial sack to form the branchial slits, but soon become entirely separated, and the two walls of each unite so as to form a broad sheet of tissue, which soon splits up to form the muscular bands of the branchial sack.” Again,p.324, “During the changes which have been described as taking place in the lateral atria, the mid-atrium has increased in size.... The branchial and atrial tunics now unite upon each side, so that the sinus is converted into a tube which communicates, at its posterior end, with the heart and perivisceral sinus, and at the anterior end with the neural sinus. This tube is the gill.... The centres of the two regions upon the sides of the gill, where these two tissues have become united, are now absorbed, so that a single long and narrow branchial slit is produced on each side of the gill. The branchial cavity is thus thrown into communication with the atrium, and the upper surface of the latter now unites with the outer tunic, and the external atrial opening is formed by absorption.”The above description would imply that the atrial cavity is a space lined by mesoblast, a view which would upset the whole morphology of the Ascidians. Salensky’s account, which implies only an immense reduction in the size of the atrial cavity as compared with other types, appears to me far more probable. The lateral atria of Brooks appear to be simply parts of the body cavity, and have certainly no connection with the lateral atria of simple Ascidians or Pyrosoma.The observations of Todaro upon Salpa (No.38) are very remarkable, and illustrated by beautifully engraved plates. His interpretations do not however appear quite satisfactory. The following is a brief statement of some of his results.During segmentation there arises a layer of small superficial cells (epiblast) and a central layer of larger cells, which becomes separated from the former by a segmentation cavity, except at the pole adjoining the free end of the brood-pouch. At this point the epiblast cells become invaginated into the central cells and form the alimentary tract, while the primitive central cells remain as the mesoblast. A fold arises from the epiblast which Todaro compares to the vertebrate amnion, but the origin of it is unfortunately not satisfactorily described. The folds of the amnion project towards the placenta, and enclose a cavity which, as the folds never completely meet, is permanently open to the maternal blood sinus. This cavity corresponds with the cavity of the true amnion of higher Vertebrates. It forms the cavity of the placenta already described. Between the two folds of the amnion is a cavity corresponding with the vertebrate false amnion. A structure regarded by Todaro as the notochord is formed on the neck, connecting the involution of the alimentary tract with the exterior. It has only a very transitory existence.In the later stages the segmentation cavity disappears and a true body cavity is formed by a split in the mesoblast.Todaro’s interpretations, and in part his descriptions also, both with reference to the notochord and amnion, appear to me quite inadmissible. About some other parts of his descriptions it is not possible to form a satisfactory judgment. He has recently published a short paper on this subject (No.39) preliminary to a larger memoir, which is very difficult to understand in the absence of plates. He finds however in the placenta various parts which he regards as homologous with the decidua vera and reflexa of Mammalia.[13]It is not within the scope of this work to enter into details with reference to the process of budding. The reader is referred on this head more especially to the papers of Huxley (No.16) and Kowalevsky (No.22) on Pyrosoma, of Salensky (No.35) on Salpa, and Kowalevsky (No.21) on Ascidians generally. It is a question of very great interest how budding first arose, and then became so prevalent in these degenerate types of Chordata. It is possible to suppose that budding may have commenced by the division of embryos at an early stage of development, and have gradually been carried onwards by the help of natural selection till late in life. There is perhaps little in the form of budding of the Ascidians to support this view—the early budding of Didemnum as described by Gegenbaur being the strongest evidence for it—but it fits in very well with the division of the embryo in Lumbricus trapezoides described by Kleinenberg, and with the not unfrequent occurrence of double monsters in Vertebrata which may be regarded as a phenomenon of a similar nature (Rauber). The embryonic budding of Pyrosoma, which might perhaps be viewed as supporting the hypothesis, appears to me not really in favour of it; since the Cyathozooid of Pyrosoma is without doubt an extremely modified form of zooid, which has obviously been specially developed in connection with the peculiar reproduction of the Pyrosomidæ.[14]The atrial spaces form somewhat doubtful exceptions to the rule.[15]Videp.33.[16]I draw this conclusion from Gegenbaur’s fig. (No.10), Pl.XVI., fig. 15. The body (x) in the figure appears to me without doubt the rudiment of the stolon, and not, as believed by Gegenbaur, the larval tail.
[5]The following classification of the Urochorda is adopted in the present chapter.
I.Caducichordata.
A. Simplicia
Solitariaex. Ascidia.
Socialiaex. Clavellina.
B. Composita
Sedentariaex. Botryllus.
Natantiaex. Pyrosoma.
C. Conserta
Salpidæ.
Doliolidæ.
II.Perennichordata.
Ex. Appendicularia.
[6]It is more probable that this part of the alimentary tract is equivalent to the postanal gut of many Vertebrata, which is at first a complete tube, but disappears later by the simple absorption of the walls.
[7]It is probable that these papillæ are very primitive organs of the Chordata. Structures, which are probably of the same nature, are formed behind the mouth in the larvæ of Amphibia, and in front of the mouth in the larvæ of Ganoids (Acipenser, Lepidosteus), and are used by these larvæ for attaching themselves.
[8]For a fuller account of the organs of sensevidethe chapters on the eye and ear.
[9]The account of the multiplication of the branchial clefts is taken from Krohn’s paper on Phallusia mammillata (No.24), but there is every reason to think that it holds true in the main for simple Ascidians.
[10]In the asexually produced buds of Ascidians the atrial cavity appears, with the exception of the external opening, to be formed from the primitive branchial sack. In the buds of Pyrosoma however it arises independently. These peculiarities in the buds cannot weigh against the embryonic evidence that the atrial cavity arises from involutions of the epiblast, and they may perhaps be partially explained by the fact that in the formation of the visceral clefts outgrowths of the branchial sack meet the atrial involutions.
[11]From Todaro’s latest paper (No.39) it would seem the segmentation cavity has very peculiar relations.
[12]Brooks takes a very different view of the nature of the parts in Salpa. He says,No.7,p.322, “The atrium of Salpa, when first observed, was composed of two broad lateral atria within the body cavity, one on each side of the branchial sack, and a very small mid-atrium.... The lateral atria do not however, as in most Tunicata, remain connected with the mid-atrium, and unite with the wall of the branchial sack to form the branchial slits, but soon become entirely separated, and the two walls of each unite so as to form a broad sheet of tissue, which soon splits up to form the muscular bands of the branchial sack.” Again,p.324, “During the changes which have been described as taking place in the lateral atria, the mid-atrium has increased in size.... The branchial and atrial tunics now unite upon each side, so that the sinus is converted into a tube which communicates, at its posterior end, with the heart and perivisceral sinus, and at the anterior end with the neural sinus. This tube is the gill.... The centres of the two regions upon the sides of the gill, where these two tissues have become united, are now absorbed, so that a single long and narrow branchial slit is produced on each side of the gill. The branchial cavity is thus thrown into communication with the atrium, and the upper surface of the latter now unites with the outer tunic, and the external atrial opening is formed by absorption.”
The above description would imply that the atrial cavity is a space lined by mesoblast, a view which would upset the whole morphology of the Ascidians. Salensky’s account, which implies only an immense reduction in the size of the atrial cavity as compared with other types, appears to me far more probable. The lateral atria of Brooks appear to be simply parts of the body cavity, and have certainly no connection with the lateral atria of simple Ascidians or Pyrosoma.
The observations of Todaro upon Salpa (No.38) are very remarkable, and illustrated by beautifully engraved plates. His interpretations do not however appear quite satisfactory. The following is a brief statement of some of his results.
During segmentation there arises a layer of small superficial cells (epiblast) and a central layer of larger cells, which becomes separated from the former by a segmentation cavity, except at the pole adjoining the free end of the brood-pouch. At this point the epiblast cells become invaginated into the central cells and form the alimentary tract, while the primitive central cells remain as the mesoblast. A fold arises from the epiblast which Todaro compares to the vertebrate amnion, but the origin of it is unfortunately not satisfactorily described. The folds of the amnion project towards the placenta, and enclose a cavity which, as the folds never completely meet, is permanently open to the maternal blood sinus. This cavity corresponds with the cavity of the true amnion of higher Vertebrates. It forms the cavity of the placenta already described. Between the two folds of the amnion is a cavity corresponding with the vertebrate false amnion. A structure regarded by Todaro as the notochord is formed on the neck, connecting the involution of the alimentary tract with the exterior. It has only a very transitory existence.
In the later stages the segmentation cavity disappears and a true body cavity is formed by a split in the mesoblast.
Todaro’s interpretations, and in part his descriptions also, both with reference to the notochord and amnion, appear to me quite inadmissible. About some other parts of his descriptions it is not possible to form a satisfactory judgment. He has recently published a short paper on this subject (No.39) preliminary to a larger memoir, which is very difficult to understand in the absence of plates. He finds however in the placenta various parts which he regards as homologous with the decidua vera and reflexa of Mammalia.
[13]It is not within the scope of this work to enter into details with reference to the process of budding. The reader is referred on this head more especially to the papers of Huxley (No.16) and Kowalevsky (No.22) on Pyrosoma, of Salensky (No.35) on Salpa, and Kowalevsky (No.21) on Ascidians generally. It is a question of very great interest how budding first arose, and then became so prevalent in these degenerate types of Chordata. It is possible to suppose that budding may have commenced by the division of embryos at an early stage of development, and have gradually been carried onwards by the help of natural selection till late in life. There is perhaps little in the form of budding of the Ascidians to support this view—the early budding of Didemnum as described by Gegenbaur being the strongest evidence for it—but it fits in very well with the division of the embryo in Lumbricus trapezoides described by Kleinenberg, and with the not unfrequent occurrence of double monsters in Vertebrata which may be regarded as a phenomenon of a similar nature (Rauber). The embryonic budding of Pyrosoma, which might perhaps be viewed as supporting the hypothesis, appears to me not really in favour of it; since the Cyathozooid of Pyrosoma is without doubt an extremely modified form of zooid, which has obviously been specially developed in connection with the peculiar reproduction of the Pyrosomidæ.
[14]The atrial spaces form somewhat doubtful exceptions to the rule.
[15]Videp.33.
[16]I draw this conclusion from Gegenbaur’s fig. (No.10), Pl.XVI., fig. 15. The body (x) in the figure appears to me without doubt the rudiment of the stolon, and not, as believed by Gegenbaur, the larval tail.
The impregnation of the ovum is effected in the oviduct. In most forms the whole of the subsequent development, till the time when the embryo is capable of leading a free existence, takes place in the uterus; but in other cases the egg becomes enveloped, during its passage down the oviduct, first in a layer of fluid albumen, and finally in a dense horny layer, which usually takes the form of a quadrilateral capsule with characters varying according to the species. After the formation of this capsule the egg is laid, and the whole of the development, with the exception of the very first stages, takes place externally.
In many of the viviparous forms (Mustelus, Galeus, Carcharias, Sphyrna) the egg is enclosed, during the early stages of development at any rate, in a very delicate shell homologous with that of the oviparous forms; there is usually also a scanty albuminous layer. Both of these are stated by Gerbe (No.42) to be absent in Squalus spinax.
The following are examples of viviparous genera: Hexanchus, Notidanus, Acanthias, Scymnus, Galeus, Squalus, Mustelus, Carcharias, Sphyrna, Squatina, Torpedo; and the following of oviparous genera: Scyllium, Pristiurus, Cestracion, Raja[17].
The ovum at the time of impregnation has the form of a large spherical mass, similar to the yolk of a bird’s egg, but without a vitelline membrane[18]. The greater part of it is formed of peculiar oval spherules of food-yolk, held together by a protoplasmic network. The protoplasm is especially concentrated in a small lens-shaped area, known as the germinal disc, which is not separated by a sharp line from the remainder ofthe ovum. Yolk spherules are present in this disc as elsewhere, but are much smaller and of a different character. The segmentation has the normal meroblastic character (fig. 15) and is confined to the germinal disc. Before it commences the germinal disc exhibits amœboid movements. During the segmentation nuclei make their appearance spontaneously (?) in the yolk adjoining the germinal disc (fig. 15,nx´), and around them portions of the yolk with its protoplasmic network become segmented off. Cells are thus formed which are added to those resulting from the segmentation proper. Even after the segmentation numerous nuclei are present in the granular matter below the blastoderm (fig. 16A,n´); and around these cells are being continually formed, which enter the blastoderm, and are more especially destined to give rise to the hypoblast. The special destination of many of these cells is spoken of in detail below.
Illustration: Figure 15Fig. 15. Section through germinal disc of a Pristiurus embryo during the segmentation.n.nucleus;nx.nucleus modified prior to division;nx´.modified nucleus in the yolk;f.furrow appearing in the yolk adjacent to the germinal disc.
Fig. 15. Section through germinal disc of a Pristiurus embryo during the segmentation.n.nucleus;nx.nucleus modified prior to division;nx´.modified nucleus in the yolk;f.furrow appearing in the yolk adjacent to the germinal disc.
At the close of segmentation the blastoderm forms a somewhat lens-shaped disc, thicker at one end than at the other; the thicker end being the embryonic end. It is divided into two strata—an upper one, the epiblast—formed of a single row of columnar cells; and a lower one, the primitive hypoblast, consisting of the remaining cells of the blastoderm, and forming a mass several strata deep. These cells will be spoken of as thelower layer cells, to distinguish them from the true hypoblast which is one of their products.
Illustration: Figure 16Fig. 16. Two longitudinal sections of the blastoderm of a Pristiurus embryo during stages prior to the formation of the medullary groove.ep.epiblast;ll.lower layer cells or primitive hypoblast;m.mesoblast;hy.hypoblast;sc.segmentation cavity;es.embryo swelling;n´.nuclei of yolk;er.embryonic rim.c.lower layer cells at the non-embryonic end of the blastoderm.
Fig. 16. Two longitudinal sections of the blastoderm of a Pristiurus embryo during stages prior to the formation of the medullary groove.ep.epiblast;ll.lower layer cells or primitive hypoblast;m.mesoblast;hy.hypoblast;sc.segmentation cavity;es.embryo swelling;n´.nuclei of yolk;er.embryonic rim.c.lower layer cells at the non-embryonic end of the blastoderm.
A cavity very soon appears in the lower layer cells, near the non-embryonic end of the blastoderm, but the cells afterwards disappear from the floor of this cavity, which then lies between the yolk and the lower layer cells (fig. 16A,sc). This cavity is the segmentation cavity equivalent to that present in Amphioxus, Amphibia, etc. The chief peculiarity about it is the relatively late period at which it makes its appearance, and the fact that its roof is formed both by the epiblast and by the lower layer cells. Owing to the large size of the segmentation cavity the blastoderm forms a thin layer above the cavity and a thickened ridge round its edge.
Illustration: Figure 17Fig. 17. Longitudinal section through the blastoderm of a Pristiurus embryo of the same age as fig. 28 B.ep.epiblast;er.embryonic rim;m.mesoblast;al.mesenteron.
Fig. 17. Longitudinal section through the blastoderm of a Pristiurus embryo of the same age as fig. 28 B.ep.epiblast;er.embryonic rim;m.mesoblast;al.mesenteron.
The epiblast in the next stage is inflected for a small arc at the embryonic end of the blastoderm, where it becomes continuous with the lower layer cells; at the same time some of the lower layer cells of the embryonic end of the blastoderm assumea columnar form, and constitute the true hypoblast. The portion of the blastoderm, where epiblast and hypoblast are continuous, forms a projecting structure which will be called the embryonic rim (fig. 16B,er).
This rim is a very important structure, since it represents the dorsal portion of the lip of the blastopore of Amphioxus. The space between it and the yolk represents the commencing mesenteron, of which the hypoblast on the under side of the lip is the dorsal wall. The ventral wall of the mesenteron is at first formed solely of yolk held together by a protoplasmic network with numerous nuclei. The cavity under the lip becomes rapidly larger (fig. 17,al), owing to the continuous conversion of lower layer cells into columnar hypoblast along an axial line passing from the middle of the embryonic rim towards the centre of the blastoderm. The continuous differentiation of the hypoblast towards the centre of the blastoderm corresponds with the invagination in Amphioxus. During the formation of the embryonic rim the blastoderm grows considerably larger, but, with the exception of the formation of the embryonic rim, retains its primitive constitution.
The segmentation cavity undergoes however important changes. There is formed below it a floor of lower layer cells, derived partly from an ingrowth from the two sides, but mainly from the formation of cells around the nuclei of the yolk (fig. 16). Shortly after the floor of cells has appeared, the whole segmentation cavity becomes obliterated (fig. 17).
The disappearance of the segmentation cavity corresponds in point of time with the formation of the hypoblast by the pseudo-invagination above described; and is probably due to this pseudo-invagination, in the same way that the disappearance of the segmentation cavity in Amphioxus is due to the true invagination of the hypoblast.
When the embryonic rim first appears there are no external indications of the embryo as distinguished from the blastoderm, but when it has attained to some importance the position of the embryo becomes marked out by the appearance of a shield-like area extending inwards from the edge of the embryonic rim, and formed of two folds with a groove between them (fig. 28B,mg), which is deepest at the edge of the blastoderm, andshallows out as it extends inwards. This groove is the medullary groove; and its termination at the edge of the blastoderm is placed at the hind end of the embryo.
At about the time of its appearance the mesoblast becomes first definitely established.
Illustration: Figure 18Fig. 18. Two transverse sections of an embryo of the same age as fig. 17.A. Anterior section.B. Posterior section.mg.medullary groove;ep.epiblast;hy.hypoblast;n.al.cells formed round the nuclei of the yolk which have entered the hypoblast;m.mesoblast.The sections shew the origin of the mesoblast.
Fig. 18. Two transverse sections of an embryo of the same age as fig. 17.A. Anterior section.B. Posterior section.mg.medullary groove;ep.epiblast;hy.hypoblast;n.al.cells formed round the nuclei of the yolk which have entered the hypoblast;m.mesoblast.The sections shew the origin of the mesoblast.
At the edge of the embryonic rim the epiblast and lower layer cells are continuous. Immediately underneath the medullary groove, as is best seen in transverse section (fig. 18), the whole of the lower layer cells become converted into hypoblast, and along this line the columnar hypoblast is in contact with the epiblast above. At the sides however this is not the case; but at the junction of the epiblast and lower layer cells the latter remain undifferentiated. A short way from the edge the lower layer cells become divided into two distinct layers, a lower one continuous with the hypoblast in the middle line, and an upper one between this and the epiblast (fig. 18B). The upper layer is the commencement of the mesoblast (m). The mesoblast thus arises as two independent lateral plates, one on each side of the medullary groove, which are continuous behind with the undifferentiated lower layer cells at the edge of the embryonic rim. The mesoblast plates are at first very short, and do not extend to the front end of the embryo. They soon however grow forwards as two lateral ridges, attached to the hypoblast, one on each side of the medullary groove (fig. 18A,m). These ridges become separate from the hypoblast, and form two plates, thinner in front than behind; but still continuous at the edge of the blastoderm with the undifferentiated cells of the lip of the blastopore, and laterally with the lower layercells of the non-embryonic part of the blastoderm. It results from the above mode of development of the mesoblast, that it may be described as arising in the form of a pair of solid outgrowths of the wall of the alimentary tract; which differ from the mesoblastic outgrowths of the wall of the archenteron in Amphioxus in not containing a prolongation of the alimentary cavity.
Illustration: Figure 19Fig. 19. Diagrammatic longitudinal sections of an Elasmobranch embryo.Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith simple shading.ep.epiblast;m.mesoblast;al.alimentary cavity;sg.segmentation cavity;nc.neural canal;ch.notochord;x.point where epiblast and hypoblast become continuous at the posterior end of the embryo;n.nuclei of yolk.A. Section of young blastoderm, with segmentation cavity enclosed in the lower layer cells.B. Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary slit has appeared. The segmentation cavity is still represented as being present, though by this stage it has in reality disappeared.C. Older blastoderm with embryo in which the neural canal has become formed, and is continuous posteriorly with the alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast.
Fig. 19. Diagrammatic longitudinal sections of an Elasmobranch embryo.Epiblastwithout shading.Mesoblastblack with clear outlines to the cells.Lower layer cellsandhypoblastwith simple shading.ep.epiblast;m.mesoblast;al.alimentary cavity;sg.segmentation cavity;nc.neural canal;ch.notochord;x.point where epiblast and hypoblast become continuous at the posterior end of the embryo;n.nuclei of yolk.A. Section of young blastoderm, with segmentation cavity enclosed in the lower layer cells.B. Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary slit has appeared. The segmentation cavity is still represented as being present, though by this stage it has in reality disappeared.C. Older blastoderm with embryo in which the neural canal has become formed, and is continuous posteriorly with the alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast.
A general idea of the structure of the blastoderm at this stage may be gathered from the diagram representing a longitudinalsection through the embryo (fig. 19B). In this figure the epiblast is represented in white and is seen to be continuous at the lip of the blastopore (x) with the shaded hypoblast. Between the epiblast and hypoblast is seen one of the lateral plates of mesoblast, represented by black cells with clear outlines. The non-embryonic lower layer cells of the blastoderm are represented in the same manner as the mesoblast of the body. The alimentary cavity is shewn atal, and below it is seen the yolk with nuclei (n). The segmentation cavity is represented as still persisting, though by this stage it would have disappeared.
Illustration: Figure 20Fig. 20. Three sections through a Pristiurus embryo somewhat younger than fig. 28 C.A. Section through the cephalic plate.B. Section through the posterior part of the cephalic plate.C. Section through the trunk.ch.notochord;mg.medullary groove;al.alimentary tract;lp.lateral plate of mesoblast;pp.body cavity.
Fig. 20. Three sections through a Pristiurus embryo somewhat younger than fig. 28 C.A. Section through the cephalic plate.B. Section through the posterior part of the cephalic plate.C. Section through the trunk.ch.notochord;mg.medullary groove;al.alimentary tract;lp.lateral plate of mesoblast;pp.body cavity.
As to the growth of the blastoderm it may be noted that it has greatly extended itself over the yolk. Its edge in the meantime forms a marked ridge, which is due not so much to a thickening as to an arching of the epiblast. This ridge is continuous with the embryonic rim, which gradually concentrates itself into two prominences, one on each side of the tail of the embryo, mainly formed of masses of undifferentiated lower layer cells. These prominences will be called the caudal swellings.
By this stage the three layers of the body, the epiblast, mesoblast, and hypoblast, have become definitely established. The further history of these layers may now be briefly traced.
Epiblast. While the greater part of the epiblast becomes converted into the external epidermis, from which involutions give rise to the olfactory and auditory pits, the lens of the eye, the mouth cavity, and anus, the part of it lining the medullary groove becomes converted into the central nervous system and optic cup. The medullary groove is at first continued to the front end of the medullary plate; but the anterior part of this plate soon enlarges, and the whole plate assumes a spatula form (fig. 28C,h, andfig. 20A and B). The enlarged part becomes converted into the brain, and may be called the cephalic plate.
The posterior part of the canal deepens much more rapidly than the rest (fig. 20C), and the medullary folds unite dorsally and convert the posterior end of the medullary groove into a closed canal, while the groove is still widely open elsewhere. The medullary canal does not end blindly behind, but simply forms a tube not closed at either extremity. The importance of this fact will appear later.
Shortly after the medullary folds have met behind the whole canal becomes closed in. This occurs in the usual way by the junction and coalescence of the medullary folds. In the course of the closing of the medullary groove the edges of the cephalic plate, which have at first a ventral curvature, become bent up in the normal manner, and enclose the dilated cephalic portion of the medullary canal. The closing of the medullary canal takes place earlier in the head and neck than in the back.
An anterior pore at the front end of the canal, like that in Amphioxus and the Ascidians, is not found. The further differentiation of the central nervous system is described in a special chapter: it may however here be stated that the walls of the medullary canal give rise not only to the central nervous system but to the peripheral also.
Mesoblast. The mesoblast was left as two lateral plates continuous behind with the undifferentiated cells of the caudal swellings.
The cells composing them become arranged in two layers (fig. 20C,lp), a splanchnic layer adjoining the hypoblast, and asomatic layer adjoining the epiblast. Between these two layers there is soon developed in the region of the head a well-marked cavity (fig. 20A,pp) which is subsequently continued into the region of the trunk, and forms the primitive body cavity, equivalent to the cavity originating as an outgrowth of the archenteron in Amphioxus. The body cavities of the two sides are at first quite independent.
Illustration: Figure 21Fig. 21. Transverse section through the trunk of an embryo slightly older than fig. 28 E.nc.neural canal;pr.posterior root of spinal nerve;x.subnotochordal rod;ao.aorta;sc.somatic mesoblast;sp.splanchnic mesoblast;mp.muscle-plate;mp´.portion of muscle-plate converted into muscle;Vv.portion of the vertebral plate which will give rise to the vertebral bodies;al.alimentary tract.
Fig. 21. Transverse section through the trunk of an embryo slightly older than fig. 28 E.
nc.neural canal;pr.posterior root of spinal nerve;x.subnotochordal rod;ao.aorta;sc.somatic mesoblast;sp.splanchnic mesoblast;mp.muscle-plate;mp´.portion of muscle-plate converted into muscle;Vv.portion of the vertebral plate which will give rise to the vertebral bodies;al.alimentary tract.
Coincidentally with the appearance of differentiation into somatic and splanchnic layers the mesoblast plates become in the region of the trunk partially split by a series of transverse lines of division into mesoblastic somites. Only the dorsal parts of the plates become split in this way, their ventral parts remaining quite intact. As a result of this each plate becomes divided into a dorsal portion adjoining the medullary canal, which is divided into somites, and may be called thevertebral plate, and a ventral portion not so divided, which may be called thelateral plate. These two parts are at this stage quite continuous with each other; and the body cavity originally extends uninterruptedly to the summit of the vertebral plates (fig. 21).
Illustration: Figure 22Fig. 22. Horizontal section through the trunk of an embryo of Scyllium considerably younger than 28 F.The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
Fig. 22. Horizontal section through the trunk of an embryo of Scyllium considerably younger than 28 F.The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
The next change results in the complete separation of the vertebral portion of the plate from the lateralportion; thereby the upper segmented part of the body cavity becomes isolated, and separated from the lower and unsegmented part. As a consequence of this change the vertebral plate comes to consist of a series of rectangular bodies, the mesoblastic somites, each composed of two layers, a somatic and a splanchnic, between which is the cavity originally continuous with the body cavity (fig. 23,mp). The splanchnic layer of the plates buds off cells to form the rudiments of the vertebral bodies which are at first segmented in the same planes as the mesoblastic somites (fig. 22,Vr). The plates themselves remain as the muscle-plates (mp), and give rise to the whole of the voluntary muscular system of the body. Between the vertebral and lateral plates there is left a connecting isthmus, with a narrow prolongation of the body cavity (fig. 23B,st), which gives rise (as described in a special chapter) to the segmental tubes and to other parts of the excretory system.
In the meantime the lateral plates of the two sides unite ventrally throughout the intestinal and cardiac regions of the body, and the two primitively isolated cavities contained in them coalesce. In the tail however the plates do not unite ventrally till somewhat later, and their contained cavities remain distinct till eventually obliterated.
At first the pericardial cavity is quite continuous with the body cavity; but it eventually becomes separated from the body cavity by the attachment of the liver to the abdominal wall, and by a horizontal septum in which run the two ductus Cuvieri (fig. 23A,sv). Two perforations in this septum (fig. 23A) leave the cavities in permanent communication.
The parts derived from the two layers of the mesoblast (not including special organs or the vascular system) are as follows:—
From the somatic layer are formed(1) A considerable part of the voluntary muscular system of the body.(2) The dermis.(3) A large part of the intermuscular connective tissue.(4) Part of the peritoneal epithelium.
From the splanchnic layer are formed(1) A great part of the voluntary muscular system.(2) Part of the intermuscular connective tissue.(3) The axial skeleton and surrounding connective tissue.(4) The muscular and connective-tissue wall of the alimentary tract.(5) Part of the peritoneal epithelium.
Illustration: Figure 23Fig. 23. Sections through the trunk of a scyllium embryo slightly younger than 28 F.Figure A shews the separation of the body cavity from the pericardial cavity by a horizontal septum in which runs the ductus Cuvieri; on the left side is seen the narrow passage which remains connecting the two cavities. Fig. B through a posterior part of the trunk shews the origin of the segmental tubes and of the primitive ova.sp.c.spinal canal;W.white matter of spinal cord;pr.commissure connecting the posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;sv.sinus venosus;cav.cardinal vein;ht.heart;pp.body cavity;pc.pericardial cavity;œs.solid œsophagus;l.liver;mp.muscle-plate;mp´.inner layer of muscle-plate;Vr.rudiment of vertebral body;st.segmental tube;sd.segmental duct;sp.v.spiral valve;v.subintestinal vein.
Fig. 23. Sections through the trunk of a scyllium embryo slightly younger than 28 F.Figure A shews the separation of the body cavity from the pericardial cavity by a horizontal septum in which runs the ductus Cuvieri; on the left side is seen the narrow passage which remains connecting the two cavities. Fig. B through a posterior part of the trunk shews the origin of the segmental tubes and of the primitive ova.sp.c.spinal canal;W.white matter of spinal cord;pr.commissure connecting the posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;sv.sinus venosus;cav.cardinal vein;ht.heart;pp.body cavity;pc.pericardial cavity;œs.solid œsophagus;l.liver;mp.muscle-plate;mp´.inner layer of muscle-plate;Vr.rudiment of vertebral body;st.segmental tube;sd.segmental duct;sp.v.spiral valve;v.subintestinal vein.
In the region of the head the mesoblast does not at first become divided into somites; but on the formation of the gill clefts a division takes place, which is apparently equivalent to the segmentation of the mesoblast in the trunk. This division causes the body cavity of the head to be divided up into a seriesof separate segments, one of which is shewn infig. 24,pp. The walls of the segments eventually give rise to the main muscles of the branchial clefts, and probably also to the muscles of the mandibular arch, of the eye, and of other parts. The cephalic sections of the body cavity will be spoken of as head cavities.
Illustration: Figure 24Fig. 24. Horizontal section through the last visceral arch but one of an embryo of Pristiurus.ep.epiblast;vc.pouch of hypoblast which will form the walls of a visceral cleft;pp.segment of body-cavity in visceral arch;aa.aortic arch.
Fig. 24. Horizontal section through the last visceral arch but one of an embryo of Pristiurus.ep.epiblast;vc.pouch of hypoblast which will form the walls of a visceral cleft;pp.segment of body-cavity in visceral arch;aa.aortic arch.
In addition to the parts already mentioned the mesoblast gives rise to the whole of the vascular system, and to the generative system. The heart is formed from part of the splanchnic mesoblast, and the generative system from a portion of the mesoblast of the dorsal part of the body cavity.
The hypoblast. Very shortly after the formation of the mesoblastic plates as lateral differentiations of the lower layer cells, an axial differentiation of the hypoblast appears, which gives rise to the notochord very much in the same way as in Amphioxus.
At first the hypoblast along the axial line forms a single layer in contact with the epiblast. Along this line a rod-like thickening of the hypoblast very soon appears (fig. 25, B and C,Ch´) at the head end of the embryo, and gradually extends backwards. This is the rudiment of the notochord; it remains attached for some time to the hypoblast, and becomes separated from it first at the head end of the embryo (fig. 25A,ch): the separation is then carried backwards.
A series of sections taken through an embryo shortly after the first differentiation of the notochord presents the following characters.
In the hindermost sections the hypoblast retains a perfectly normal structure and uniform thickness throughout. In the next few sections (fig. 25C,Ch´) a slight thickening is to be observed in it, immediately below the medullary groove. The layer, which elsewhere is composed of a single row of cells, here becomes two cells deep, but no sign of a division into two layers is exhibited.
In the next few sections the thickening of the hypoblast becomes much more pronounced; we have, in fact, a ridge projecting from the hypoblast towards the epiblast (fig. 25B,Ch´). This ridge is pressed firmly againstthe epiblast, and causes in it a slight indentation. The hypoblast in the region of the ridge is formed of two layers of cells, the ridge being entirely due to the uppermost of the two.
Illustration: Figure 25Fig. 25. Three sections of a Pristiurus embryo slightly older than fig. 28 B.The sections shew the development of the notochord.Ch.notochord;Ch´.developing notochord;mg.medullary groove;lp.lateral plate of mesoblast;ep.epiblast;hy.hypoblast.
Fig. 25. Three sections of a Pristiurus embryo slightly older than fig. 28 B.The sections shew the development of the notochord.Ch.notochord;Ch´.developing notochord;mg.medullary groove;lp.lateral plate of mesoblast;ep.epiblast;hy.hypoblast.
In sections in front of this a cylindrical rod, which can at once be recognized as the notochord, and is continuous with the ridge just described, begins to be split off from the hypoblast (fig. 25A,Ch). It is difficult to say at what point the separation of this rod from the hypoblast is completed, since all intermediate gradations between complete separation and complete attachment are to be seen.
Shortly after the separation takes place, a fairly thick bridge is found connecting the two lateral halves of the hypoblast, but this bridge is anteriorly excessively delicate and thin, and in some cases is barely visible except with high powers. In some sections I have observed possible indications of the process like that described by Calberla for Petronyzon, by which the lateral parts of the hypoblast grow in underneath the axial part, and so isolate it bodily as the notochord.
It is not absolutely clear whether the notochord is to be regarded as an axial differentiation of the hypoblast, or as an axial differentiation of the lower layer cells.
The facts of development both in Amphioxus and Elasmobranchii tend towards the former view; but the nearly simultaneous differentiation of the notochord and the mesoblastic plates lends some support to the supposition that the notochord may be merely a median plate of mesoblast developed slightly later than the two lateral plates.
The alimentary canal or mesenteron was left as a space between the hypoblast and the yolk, ending blindly in front, butopening behind by a widish aperture, the blastopore or anus of Rusconi (videfig. 19B).
Illustration: Figure 26Fig. 26. Section through the anterior part of a Pristiurus embryo to shew the formation of the alimentary tract.Ch.notochord;hy.hypoblast;al.alimentary tract;na.cells passing in from the yolk to form the ventral wall of the alimentary tract.
Fig. 26. Section through the anterior part of a Pristiurus embryo to shew the formation of the alimentary tract.Ch.notochord;hy.hypoblast;al.alimentary tract;na.cells passing in from the yolk to form the ventral wall of the alimentary tract.
The conversion of this irregular cavity into a closed canal commences first of all at the anterior extremity. In this conversion two distinct processes are concerned. One of these is a process of folding off of the embryo from the blastoderm. The other is a simple growth of cells independent of any fold. To the first of these processes the depth and narrowness of the alimentary cavity is due; the second is concerned in forming its ventral wall. The process of the folding off of the embryo from the blastoderm resembles exactly the similar process in the embryo bird. The fold is a perfectly continuous one round the front end of the embryo, but may be conveniently spoken of as composed of a head-fold and two lateral folds.