Illustration: Figure 162Fig. 162. Diagrammatic longitudinal section through the embryo of a Guinea-pig with its membranes.(After Schäfer.)e.epiblast;h.hypoblast;m´.amniotic mesoblast;m´´.splanchnic mesoblast;am.amnion;ev.cavity of amnion;all.allantois;f.rudimentary blastopore;mc.cavity of vesicle continuous with body cavity;mm.mucous membrane of uterus;m´m´.parts where vascular uterine tissue perforates hypoblast of blastodermic vesicle;vt.uterine vascular tissue;l.limits of uterine tissue.
Fig. 162. Diagrammatic longitudinal section through the embryo of a Guinea-pig with its membranes.(After Schäfer.)e.epiblast;h.hypoblast;m´.amniotic mesoblast;m´´.splanchnic mesoblast;am.amnion;ev.cavity of amnion;all.allantois;f.rudimentary blastopore;mc.cavity of vesicle continuous with body cavity;mm.mucous membrane of uterus;m´m´.parts where vascular uterine tissue perforates hypoblast of blastodermic vesicle;vt.uterine vascular tissue;l.limits of uterine tissue.
On this day a cavity develops in the interior of this body which at the same time enlarges itself. The greater part of its wall next attaches itself to the free end of the cylinder, and becomes considerably thickened. The remainder of the wall adjoining the cavity of the cylinder becomes a comparatively thin membrane. At the free end of the cylinder there appears on the thirteenth day an embryonic area similar to that of other Mammalia. It is at first round but soon becomes pyriform, and in it there appear a primitive streak and groove; and on their appearance it becomes obviousthat the outer layer of the cylinder is the hypoblast[95],instead of, as in all other Mammalia, the epiblast; and that the epiblast is formed by the wall of the inner vesicle, i.e. the original solid body placed at the end of the cylinder. Thus the dorsal surface of the embryo is turned inwards, and the ventral surface outwards, and the ordinary position of the layers is completely inverted.
The previously cylindrical egg next assumes a spherical form, and the mesoblast arises in connection with the primitive streak in the manner already described. A splanchnic layer of mesoblast attaches itself to the inner side of the outer hypoblastic wall of the egg, a somatic layer to the epiblast of the inner vesicle, and a mass of mesoblast grows out into the cavity of the larger vesicle forming the commencement of the allantois. The general structure of the ovum at this stage is represented onfig. 162, copied from Schäfer; and the condition of the whole ovum will best be understood by a description of this figure.
It is seen to consist of two vesicles, (1) an outer larger one (h)—the original egg-cylinder—united to the mesometric wall of the uterus by a vascular connection atm´m´, and (2) an inner smaller one (ev)—the originally solid body at the free end of the egg-cylinder. The outer vesicle is formed of (1) an external lining of columnar hypoblast (h) which is either pierced or invaginated at the area of vascular connection with the uterus, and (2) of an inner layer of splanchnic mesoblast (m´´) which covers without a break the vascular uterine growth. At the upper pole of the ovum is placed the smaller epiblastic vesicle, and where the two vesicles come together is situated the embryonic area with the primitive streak (f), and the medullary plate seen in longitudinal section. The thinner wall of the inner vesicle is formed of epiblast and somatic mesoblast, and covers over the dorsal face of the embryo just like the amnion. It is in fact usually spoken of as the amnion. The large cavity of the outer vesicle is continuous with the body cavity, and into it projects the solid mesoblastic allantois (all), so far without hypoblast[96].
The outer vesicle corresponds exactly with the yolk-sack, and its mesoblastic layer receives the ordinary vascular supply.
The embryo becomes folded off from the yolk-sack in the usual way, but comes to lienot outsideit as in the ordinary form, butin its interior, and is connected with it by an umbilical stalk. The yolk-sack forms the substitute for part of the subzonal membrane of other Mammalia. The so-called amnion appears to me from its development and position rather to correspond with the non-embryonic part of the epiblastic wall (true subzonal membrane) of the blastodermic vesicle of the ordinary mammalian forms than with the true amnion; and a true amnion would seem not to be developed.
The allantois meets the yolk-sack on about the seventeenth day at the region of its vascular connection with the uterine wall, and gives rise to the placenta. A diagrammatic representation of the structure of the embryo at this stage is given infig. 163.
The peculiar inversion of the layers in the Guinea-pig has naturally excited the curiosity of embryologists, but as yet no satisfactory explanation has been offered of it.
At the time when the ovum first becomes fixed it will be remembered that it resembles the early blastodermic vesicle of the Rabbit, and it is natural to suppose that the apparently hypoblastic mass attached to the inner wall of the vesicle becomes the solid body at the end of the egg-cylinder. This appears to be Bischoff’s view, but, as shewn above, the solid mass is really the epiblast! Is it conceivable that the hypoblast in one species becomes the epiblast in a closely allied species? To my mind it is not conceivable, and I am reduced to the hypothesis, put forward by Hensen, that in the course of the attachment of the ovum to the wall of the uterus a rupture of walls of the blastodermic vesicle takes place, and that they become completely turned inside out. It must be admitted, however, that in the present state of our knowledge of the development of the ovum on the seventh and eighth days it is not possible to frame a satisfactory explanation how such an inversion can take place.
Illustration: Figure 163Fig. 163. Diagrammatic longitudinal section of an ovum of a Guinea-pig and the adjacent uterine walls at an advanced stage of pregnancy.(After Bischoff.)yk.inverted yolk-sack (umbilical vesicle) formed of an external hypoblastic layer (shaded) and an internal vascular layer (black). At the end of this layer is placed the sinus terminalis;all.allantois;pl.placenta.The external shaded parts are the uterine walls.
Fig. 163. Diagrammatic longitudinal section of an ovum of a Guinea-pig and the adjacent uterine walls at an advanced stage of pregnancy.(After Bischoff.)yk.inverted yolk-sack (umbilical vesicle) formed of an external hypoblastic layer (shaded) and an internal vascular layer (black). At the end of this layer is placed the sinus terminalis;all.allantois;pl.placenta.The external shaded parts are the uterine walls.
The Human Embryo. Our knowledge as to the early development of the human embryo is in an unsatisfactory state. The positive facts we know are comparatively few, and it is not possible to construct from them a history of the development which is capable of satisfactory comparison with that in other forms, unless all the early embryos known are to be regarded as abnormal. The most remarkable feature in the development, which was first clearly brought to light by Allen Thomson in 1839, is the very early appearance of branched villi. In the last few years several ova, even younger than those described by Allen Thomson, have been met with, which exhibit this peculiarity.
The best-preserved of these ova is one described by Reichert (No.237). This ovum, though probably not more than thirteen days old, was completely enclosed by a decidua reflexa. It had (fig. 164A and B) a flattened oval form, measuring in its two diameters 5.5mm.and 3.5mm.The edge was covered with branched villi, while in the centre of each of the flattened surfaces there was a spot free from villi. On the surface adjoining the uterine wall was a darker area (e) formed of two layers of cells, which is interpreted by Reichert as the embryonic area, while the membrane formingthe remainder of the ovum, including the branched villi, was stated by Reichert to be composed of a single row of epithelial cells.
Whether or no Reichert is correct in identifying his darker spot as the embryonic area, it is fairly certain from the later observations of Beigel and Löwe (No.228), Ahlfeld (No.227), and Kollmann (No.234) on ova nearly as young as that of Reichert, that the wall of very young ova has a more complicated structure than Reichert is willing to admit. These authors do not however agree amongst themselves, but from Kollmann’s description, which appears to me the most satisfactory, it is probable that it is composed of an outer epithelial layer, and an inner layer of connective tissue, and that the connective tissue extends at a very early period into the villi; so that the latter are not hollow, as Reichert supposed them to be.
Illustration: Figure 164Fig. 164. The human ova during early stages of development.(From Quain’sAnatomy.)A. and B. Front and side view of an ovum figured by Reichert, supposed to be about thirteen days.e.embryonic area.C. An ovum of about four or five weeks shewing the general structure of the ovum before the formation of the placenta. Part of the wall of the ovum is removed to shew the embryoin situ. (After Allen Thomson.)
Fig. 164. The human ova during early stages of development.(From Quain’sAnatomy.)A. and B. Front and side view of an ovum figured by Reichert, supposed to be about thirteen days.e.embryonic area.C. An ovum of about four or five weeks shewing the general structure of the ovum before the formation of the placenta. Part of the wall of the ovum is removed to shew the embryoin situ. (After Allen Thomson.)
The villi, which at first leave the flattened poles free, seem soon to extend first over one of the flat sides, and finally over the whole ovum (fig. 164C).
Unless the two-layered region of Reichert’s ovum is the embryonic area, nothing which can clearly be identified as an embryo has been detected in these early ova. In an ovum described by Breus (No.228), and in one described long ago by Wharton-Jones a mass found in the interior of the egg may perhaps be interpreted (His) as the remains of the yolk. It is, however, very probable that all the early ova so far discovered are more or less pathological.
The youngest ovum with a distinct embryo is one described by His (No.232). This ovum, which is diagrammatically represented infig. 168in longitudinal section, had the form of an oval vesicle completely covered by villi, and about 8.5mm.and 5.5mm.in its two diameters, and flatter on one side than on the other. An embryo with a yolk-sack was attached to the inner side of the flatter wall of the vesicle by a stalk, which must beregarded as the allantoic stalk[97], and the embryo and yolk-sack filled up but a very small part of the whole cavity of the vesicle.
The embryo, which was probably not quite normal (fig. 165A), was very imperfectly developed; a medullary plate was hardly indicated, and, though the mesoblast was unsegmented, the head fold, separating the embryo from the yolk-sack (um), was already indicated. The amnion (am) was completely formed, and vitelline vessels had made their appearance.
Illustration: Figure 165Fig. 165. Three early human embryos.(Copied from His.)A. An early embryo described by His from the side.am.amnion;um.umbilical vesicle;ch.chorion, to which the embryo is attached by a stalk.B. Embryo described by Allen Thomson about 12-14 days.um.umbilical vesicle;md.medullary groove.C. Young embryo described by His.um.umbilical vesicle.
Fig. 165. Three early human embryos.(Copied from His.)A. An early embryo described by His from the side.am.amnion;um.umbilical vesicle;ch.chorion, to which the embryo is attached by a stalk.B. Embryo described by Allen Thomson about 12-14 days.um.umbilical vesicle;md.medullary groove.C. Young embryo described by His.um.umbilical vesicle.
Two embryos described by Allen Thomson (No.239) are but slightly older than the above embryos of His. Both of them probably belong to the first fortnight of pregnancy. In both cases the embryo was more or less folded off from the yolk-sack, and in one of them the medullary groove was still widely open, except in the region of the neck (fig. 165B). The allantoic stalk, if present, was not clearly made out, and the condition of the amnion was also not fully studied. The smaller of the two ova was just 6mm.inits largest diameter, and was nearly completely covered with simple villi, more developed on one side than on the other.
In a somewhat later period, about the stage of a chick at the end of the second day, the medullary folds are completely closed, the region of the brain already marked, and the cranial flexure commencing. The mesoblast is divided up into numerous somites, and the mandibular and first two branchial arches are indicated. The embryo is still but incompletely folded off from the yolk-sack below.
In a still older stage the cranial flexure becomes still more pronounced, placing the mid-brain at the end of the long axis of the body. The body also begins to be ventrally curved (fig. 165C).
Externally human embryos at this age are characterised by the small size of the anterior end of the head.
Illustration: Figure 166Fig. 166. Two views of a human embryo of between the third and fourth week.A. Side view. (From Kölliker; after Allen Thomson.)a.amnion;b.umbilical vesicle;c.mandibular arch;e.hyoid arch;f.commencing anterior limb;g.primitive auditory vesicle;h.eye;i.heart.B. Dorsal view to shew the attachment of the dilated allantoic stalk to the chorion. (From a sketch by Allen Thomson.)am.amnion;all.allantois;ys.yolk-sack.
Fig. 166. Two views of a human embryo of between the third and fourth week.A. Side view. (From Kölliker; after Allen Thomson.)a.amnion;b.umbilical vesicle;c.mandibular arch;e.hyoid arch;f.commencing anterior limb;g.primitive auditory vesicle;h.eye;i.heart.B. Dorsal view to shew the attachment of the dilated allantoic stalk to the chorion. (From a sketch by Allen Thomson.)am.amnion;all.allantois;ys.yolk-sack.
The flexure goes on gradually increasing, and in the third week of pregnancy in embryos of about 4mm.the limbs make their appearance. The embryo at this stage (fig. 166), which is about equivalent to that of a chick on the fourth day, resembles in almost every respect the normal embryos of the Amniota. The cranial flexure is as pronounced as usual, and the cerebral region has now fully the normal size. The whole body soon becomes flexed ventrally, and also somewhat spirally. The yolk-sack (b) forms a small spherical appendage with a long wide stalk, and the embryo (B) is attached by an allantoic stalk with a slight swelling (all), probably indicating the presence of a small hypoblastic diverticulum, to the inner face of the chorion.
A remarkable exception to the embryos generally observed is afforded by an embryo which has been described by Krause (No.235). In thisembryo, which probably belongs to the third week of pregnancy, the limbs were just commencing to be indicated, and the embryo was completely covered by an amnion, but instead of being attached to the chorion by an allantoic cord, it was quite free, and was provided with a small spherical sack-like allantois, very similar to that of a fourth-day chick, projected from its hind end.
Illustration: Figure 167Fig. 167. Figures shewing the early changes in the form of the human head.(From Quain’sAnatomy.)A. Head of an embryo of about four weeks. (After Allen Thomson.)B. Head of an embryo of about six weeks. (After Ecker.)C. Head of an embryo of about nine weeks.1. mandibular arch; 1´. persistent part of hyomandibular cleft;a.auditory vesicle.
Fig. 167. Figures shewing the early changes in the form of the human head.(From Quain’sAnatomy.)A. Head of an embryo of about four weeks. (After Allen Thomson.)B. Head of an embryo of about six weeks. (After Ecker.)C. Head of an embryo of about nine weeks.1. mandibular arch; 1´. persistent part of hyomandibular cleft;a.auditory vesicle.
No details are given as to the structure of the chorion or the presence of villi upon it. The presence of such an allantois at this stage in a human embryo is so unlike what is usually found that Krause’s statements have been received with considerable scepticism. His even holds that the embryo is a chick embryo, and not a human one; while Kölliker regards Krause’s allantois as a pathological structure. The significance to be attached to this embryo is dealt with below.
A detailed history of the further development of the human embryo does not fall within the province of this work; while the later changes in the embryonic membranes have already been dealt with (pp.244-248).
For the changes which take place on the formation of the face I may refer the reader tofig. 167.
The most obscure point connected with the early history of the human ovum concerns the first formation of the allantois, and the nature of the villi covering the surface of the ovum. The villi, if really formed of mesoblast covered by epiblast, have the true structure of chorionic villi; and can hardly be compared to the early villi of the dog which are derived from the subzonal membrane, and still less to those of the rabbit formed from the zona radiata.
Unless all the early ova so far described are pathological, it seems tofollow that the mesoblast of the chorion is formed before the embryo is definitely established, and even if the pathological character of these ova is admitted, it is nevertheless probable (leaving Krause’s embryo out of account), as shewn by the early embryos of Allen Thomson and His, that it is formed before the closure of the medullary groove. In order to meet this difficulty His supposes that the embryo never separates from the blastodermic vesicle, but that the allantoic stalk of the youngest embryo (fig. 168) represents the persistent attachment between the two[98]. His’ view has a good deal to be said for it. I would venture, however, to suggest that Reichert’s embryonic area is probably not in the two-layered stage, but that a mesoblast has already become established, and that it has grown round the inner face of the blastodermic vesicle from the (apparent) posterior end of the primitive streak. This growth I regard asa precocious formation of the mesoblast of the allantois—an exaggeration of the early formation of the allantoic mesoblast which is characteristic of the Guinea-pig (videp.264). This mesoblast, together with the epiblast, forms a true chorion, so that infig. 168, and probably also infig. 164A and B, a true chorion has already become established. The stalk connecting the embryo with the chorion in His’ earliest embryo (fig. 168) is therefore a true allantoic stalk into which the hypoblastic allantoic diverticulum grows in for some distance. How the yolk-sack (umbilical vesicle) is formed is not clear. Perhaps, as suggested by His, it arises from the conversion of a solid mass of primitive hypoblast directly into a yolk-sack. The amnion is probably formed as a fold over the head end of the embryo in the manner indicated in His’ diagram (fig. 168Am).
Illustration: Figure 168Fig. 168. Diagrammatic longitudinal section of the ovum to which the embryo (fig. 165a) belonged.(After His.)Am.amnion;Nb.umbilical vesicle.
Fig. 168. Diagrammatic longitudinal section of the ovum to which the embryo (fig. 165a) belonged.(After His.)Am.amnion;Nb.umbilical vesicle.
These speculations have so far left Krause’s embryo out of account. How is this embryo to be treated? Krause maintains that all the other embryos shewing an allantoic stalk at an early age are pathological. This, though not impossible, appears to me, to say the least of it, improbable; especially when it is borne in mind that embryos, which have every appearance of being normal, of about the same age and younger than Krause’s, have been frequently observed, and have always been found attached to the chorion by an allantoic stalk.
We are thus provisionally reduced to suppose either that the structure figured by Krause is not the allantois, or that it is a very abnormal allantois. It is perhaps just possible that it may be an abnormally developed hypoblastic vesicle of the allantois artificially detached from the mesoblastic layer,—the latter having given rise to the chorion at an earlier date.
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[76]It is stated by Bischoff that shortly after impregnation, and before the commencement of the segmentation, the ova of the rabbit and guinea-pig are covered with cilia and exhibit the phenomenon of rotation. This has not been noticed by other observers.[77]Van Beneden regards it as probable that the blastopore is situated somewhat excentrically in relation to the area of attachment of the hypoblastic mass to the epiblast.[78]The attempt made below to frame a consecutive history out of the contradictory data at my disposal is not entirely satisfactory. Should Kölliker’s view turn out to be quite correct, the origin of the middle layer of the fifth day, which Kölliker believes to become the permanent epiblast, will have to be worked out again, in order to determine whether it really comes, as it is stated by Van Beneden to do, from the primitive hypoblast.[79]The section figured may perhaps hardly appear to justify this view; the examination of a larger number of sections is, however, more favourable to it, but it must be admitted that the interpretation is by no means thoroughly satisfactory.[80]Kölliker does not believe in the existence of this stage, having never met with it himself. It appears to me, however, more probable that Kölliker has failed to obtain it, than that Van Beneden has been guilty of such an extraordinary blunder as to have described a stage which has no existence.[81]Schäfer describes the blastodermic vesicle of the cat as being throughout in a bilaminar condition before the formation of a definite primitive streak or of the mesoblast.[82]This figure was drawn for me by my pupil, Mr Weldon.[83]The hypoblastic element in the allantois is sometimes very much reduced, so that the allantois may be mainly formed of a vascular layer of mesoblast.[84]These crypts have no connection with the openings of glands in the walls of the uterus. They are believed by Ercolani to be formed to a large extent by a regeneration of the lining tissue of the uterine walls.[85]The following is Owen’s account of the young after birth (Comp. Anat. of Vertebrates,Vol.III.p.717): “On the eighth of December Dr Bennet discovered in the subterranean nest of Ornithorhyncus three living young, naked, not quite two inches in length.” On the 12th of August, 1864, “a female Echidna hystrix was captured ... having a young one with its head buried in a mammary or marsupial fossa. This young one was naked, of a bright red colour, and one inch two lines in length.”[86]Owen quotes in theAnatomy of Vertebrates,Vol.III. p. 721, a description from Rengger of the development of Didelphis azaræ, which would seem to imply that a vascular adhesion arises between the uterine walls and the subzonal membrane, but the description is too vague to be of any value in determining the nature of the fœtal membranes.[87]Numerous contributions to our knowledge of the various types of placenta have been made during the last few years, amongst which those of Turner and Ercolani may be singled out, both from the variety of forms with which they deal, and the important light they have thrown on the structure of the placenta.[88]VideErcolani,No.197, and Harting,No.201, and also Von Baer,Entwicklungsgeschichtetable onp.225, partI., where the importance of the limited area of attachment of the allantois as compared with the yolk-sack is distinctly recognised.[89]This is denied by Nasse;videKölliker,No.183,p.361.[90]VideBischoff,No.175.[91]According to Bischoff the subzonal membrane atrophies, leaving the allantoic mesoblast to constitute the whole chorion.[92]The observations on this head were made by Sharpey, and are quoted by Huxley (No.202) and with additional observations by Turner in his Memoir on the placentation of the Sloths. Anderson (No.191) has also recently confirmed Sharpey’s account of the diffused character of the placenta of Manis.[93]Entwicklungsgeschichte des Menschen,etc.,2nd ed., p.362. Leipzig, 1876.[94]Schäfer’s and Hensen’s statements are in more or less direct contradiction as to the structure of the ovum after the formation of the embryo; and it is not possible to decide between the two views about the ovum till these points of difference have been cleared up.[95]According to Hensen the hypoblast grows round the inside of the wall of the cylinder from the body which he regards as the ovum. The original wall of the cylinder persists as a very thin layer separated from the hypoblast by a membrane.[96]Hensen states that the hypoblast never grows into the allantois; while Bischoff, though not very precise on the point, implies that it does; he states however that it soon disappears.[97]Allen Thomson informs me that he is very confident that such a form of attachment between the hind end of the embryo and the wall of the vesicle, as that described and figured by His in this embryo, did not exist in any of the younger embryos examined by him.[98]For a fuller explanation of His’ views I must refer the reader to his Memoir (No.232),pp.170, 171, and to the diagrams contained in it.
[76]It is stated by Bischoff that shortly after impregnation, and before the commencement of the segmentation, the ova of the rabbit and guinea-pig are covered with cilia and exhibit the phenomenon of rotation. This has not been noticed by other observers.
[77]Van Beneden regards it as probable that the blastopore is situated somewhat excentrically in relation to the area of attachment of the hypoblastic mass to the epiblast.
[78]The attempt made below to frame a consecutive history out of the contradictory data at my disposal is not entirely satisfactory. Should Kölliker’s view turn out to be quite correct, the origin of the middle layer of the fifth day, which Kölliker believes to become the permanent epiblast, will have to be worked out again, in order to determine whether it really comes, as it is stated by Van Beneden to do, from the primitive hypoblast.
[79]The section figured may perhaps hardly appear to justify this view; the examination of a larger number of sections is, however, more favourable to it, but it must be admitted that the interpretation is by no means thoroughly satisfactory.
[80]Kölliker does not believe in the existence of this stage, having never met with it himself. It appears to me, however, more probable that Kölliker has failed to obtain it, than that Van Beneden has been guilty of such an extraordinary blunder as to have described a stage which has no existence.
[81]Schäfer describes the blastodermic vesicle of the cat as being throughout in a bilaminar condition before the formation of a definite primitive streak or of the mesoblast.
[82]This figure was drawn for me by my pupil, Mr Weldon.
[83]The hypoblastic element in the allantois is sometimes very much reduced, so that the allantois may be mainly formed of a vascular layer of mesoblast.
[84]These crypts have no connection with the openings of glands in the walls of the uterus. They are believed by Ercolani to be formed to a large extent by a regeneration of the lining tissue of the uterine walls.
[85]The following is Owen’s account of the young after birth (Comp. Anat. of Vertebrates,Vol.III.p.717): “On the eighth of December Dr Bennet discovered in the subterranean nest of Ornithorhyncus three living young, naked, not quite two inches in length.” On the 12th of August, 1864, “a female Echidna hystrix was captured ... having a young one with its head buried in a mammary or marsupial fossa. This young one was naked, of a bright red colour, and one inch two lines in length.”
[86]Owen quotes in theAnatomy of Vertebrates,Vol.III. p. 721, a description from Rengger of the development of Didelphis azaræ, which would seem to imply that a vascular adhesion arises between the uterine walls and the subzonal membrane, but the description is too vague to be of any value in determining the nature of the fœtal membranes.
[87]Numerous contributions to our knowledge of the various types of placenta have been made during the last few years, amongst which those of Turner and Ercolani may be singled out, both from the variety of forms with which they deal, and the important light they have thrown on the structure of the placenta.
[88]VideErcolani,No.197, and Harting,No.201, and also Von Baer,Entwicklungsgeschichtetable onp.225, partI., where the importance of the limited area of attachment of the allantois as compared with the yolk-sack is distinctly recognised.
[89]This is denied by Nasse;videKölliker,No.183,p.361.
[90]VideBischoff,No.175.
[91]According to Bischoff the subzonal membrane atrophies, leaving the allantoic mesoblast to constitute the whole chorion.
[92]The observations on this head were made by Sharpey, and are quoted by Huxley (No.202) and with additional observations by Turner in his Memoir on the placentation of the Sloths. Anderson (No.191) has also recently confirmed Sharpey’s account of the diffused character of the placenta of Manis.
[93]Entwicklungsgeschichte des Menschen,etc.,2nd ed., p.362. Leipzig, 1876.
[94]Schäfer’s and Hensen’s statements are in more or less direct contradiction as to the structure of the ovum after the formation of the embryo; and it is not possible to decide between the two views about the ovum till these points of difference have been cleared up.
[95]According to Hensen the hypoblast grows round the inside of the wall of the cylinder from the body which he regards as the ovum. The original wall of the cylinder persists as a very thin layer separated from the hypoblast by a membrane.
[96]Hensen states that the hypoblast never grows into the allantois; while Bischoff, though not very precise on the point, implies that it does; he states however that it soon disappears.
[97]Allen Thomson informs me that he is very confident that such a form of attachment between the hind end of the embryo and the wall of the vesicle, as that described and figured by His in this embryo, did not exist in any of the younger embryos examined by him.
[98]For a fuller explanation of His’ views I must refer the reader to his Memoir (No.232),pp.170, 171, and to the diagrams contained in it.
Although the preceding chapters of this volume contain a fairly detailed account of the early developmental stages of different groups of the Chordata, it will nevertheless be advantageous to give at this place a short comparative review of the whole subject.
In this review only the most important points will be dwelt upon, and the reader is referred for the details of the processes to the sections on the development of the individual groups.
The subject may conveniently be treated under three heads.(1) The formation of the gastrula and behaviour of the blastopore: together with the origin of the hypoblast.(2) The mesoblast and notochord.(3) The epiblast.
At the close of the chapter is a short summary of the organs derived from the several layers, together with some remarks on the growth in length of the vertebrate embryo, and some suggestions as to the origin of the allantois and amnion.
Formation of the gastrula.Amphioxus is the type in which the developmental phenomena are least interfered with by the presence of food-yolk.
In this form the segmentation results in a uniform, or nearly uniform, blastosphere, one wall of which soon becomes thickened and invaginated, giving rise to the hypoblast; while the larva takes the form of a gastrula, with an archenteric cavity opening by a blastopore. The blastopore rapidly narrows, while theembryo assumes an elongated cylindrical form with the blastopore at its hinder extremity (fig. 169A). The blastopore now passes to the dorsal surface, and by the flattening of this surface a medullary plate is formed extending forwards from the blastopore (fig. 169B). On the formation of the medullary groove and its conversion into a canal, the blastopore opens into this canal, and gives rise to a neurenteric passage, leading from the neural canal into the alimentary tract (fig. 169C and E). At a later period this canal closes, and the neural and alimentary canals become separated.