Illustration: Figure 83Fig. 83. Larva of Dactylethra.(After Parker.)
Fig. 83. Larva of Dactylethra.(After Parker.)
Although most Anurous embryos are not provided with a sufficient amount of yolk to give rise to a yolk-sack as an external appendage of the embryo, yet in some forms a yolk-sack, nearly as large as that of Teleostei, is developed. One of these forms, Alytes obstetricans, belongs to a well-known European genus allied to Pelobates. The embryos of Pipa dorsigera (Parker) are also provided with a very large yolk-sack, round which they are coiled like a Teleostean embryo. A large yolk-sack is also developed in the embryo of Pseudophryne australis.
The actual complexity of the organization of different tadpoles, and their relative size, as compared with the adult, vary considerably. The tadpoles of Toads are the smallest, Pseudophryne australis excelling in this respect; those of Pseudis are the largest known.
The external gills reach in certain forms, which are hatched in late larval stages, a very great development. It seems however that this development is due to these gills being especially required in the stages before hatching. Thus in Alytes, in which the larva leaves the egg in a stage after the loss of the external gills, these structures reach in the egg a very great development. In Notodelphis ovipara, in which the eggs are carried in a dorsal pouch of the mother, the embryos are provided with long vesicular gills attached to the neck by delicate threads. The fact (if confirmed) that some of the forms which are not hatched till post-larval stages are without external gills, probably indicates that there may be various contrivances for embryonic respiration[55]; and that the external gills only attain a great development inthose instances in which respiration is mainly carried on by their means. The external gills of Elasmobranchii are probably, as stated in a previous chapter, examples of secondarily developed structures, which have been produced by the same causes as the enlarged gills of Alytes, Notodelphis, etc.
Urodela. Up to the present time complete observations on the development of the Urodela are confined to the Myctodera[56].
The early stages are in the main similar to those of the Anura. The body of the embryo is, as pointed out by Scott and Osborn, ventrally instead of dorsally flexed. The metamorphosis is much less complete than in the Anura. The larva of Triton may be taken as typical. At hatching, it is provided with a powerful swimming tail bearing a well-developed fin: there are three pairs of gills placed on the three anterior of the true branchial arches.
Between the hyoid and first branchial arch, and between the other branchial arches, slits are developed, there being four slits in all. At the period just before hatching, only three of these have made their appearance. The hyomandibular cleft is not perforated. Stalked suckers, of the same nature as the suckers of the Anura, are formed on the ventral surface behind the mouth. A small opercular fold, developed from the lower part of the hyoid arch, covers over the bases of the gills. The suctorial mouth and the provisional horny beak of the Anura have no counterpart in these larvæ. The skin is ciliated, and the cilia cause a rotation in the egg. Even before hatching, a small rudiment of the anterior pair of limbs is formed, but the hind-limbs are not developed till a later stage, and the limbs do not attain to any size till the larva is well advanced. In the course of the subsequent metamorphosis lungs become developed, and a pulmonary respiration takes the place of the branchial one. The branchial slits at the same time close and the branchiæ atrophy.
The other types of Myctodera, so far investigated, agree fairly with the Newt.
The larva of Amblystoma punctatum (fig. 84) is provided with two verylong processes (s), like the suctorial processes in Triton, placed on the throat in front of the external gills. They are used to support the larva when it sinks to the bottom, and have been called by Clarke (No.98) balancers. On the development of the limbs, these processes drop off. The external gills atrophy about one hundred days after hatching.
It might have been anticipated that the Axolotl, being a larval form of Amblystoma, would agree in development with Amblystoma punctatum. The conspicuous suctorial processes of the latter form are however represented by the merest rudiments in the Axolotl.
Illustration: Figure 84Fig. 84. Larvæ of Amblystoma punctatum.(After Clarke.)n.nasal pit;f.oral invagination;op.eye;s.balancers;f.l.front limb;br.branchiæ.
Fig. 84. Larvæ of Amblystoma punctatum.(After Clarke.)n.nasal pit;f.oral invagination;op.eye;s.balancers;f.l.front limb;br.branchiæ.
The young of Salamandra maculata leave the uterus with external gills, but those of the Alpine Salamander (Salamandra atra) are born in the fully developed condition without gills. In the uterus they pass through a metamorphosis, and are provided (in accordance with the principle already laid down) with very long gill-filaments[57].
Salamandra atra has only two embryos, but there are originally a larger number of eggs (Von Siebold), of which all but two fail to develop, while their remains are used as pabulum by the two which survive. Both species of Salamander have a sufficient quantity of food-yolk to give rise to a yolk-sack.
Spelerpes only develops three post-hyoid arches, between which slits are formed as in ordinary types. Menobranchus and Proteus agree with Spelerpes in the number of post-hyoid arches.
One of the most remarkable recent discoveries with reference to the metamorphosis of the Urodela was made by Dumeril[58]. He found that some of the larvæ of the Axolotl, bred in the Jardin des Plantes, left the water, and in the course of about a fortnight underwent a similar metamorphosis to that of the Newt, and became converted into a form agreeing in everyparticular with the American genus Amblystoma. During this metamorphosis a pulmonary respiration takes the place of a branchial one, the gills are lost, and the gill slits close. The tail loses its fin and becomes rounded, the colour changes, and alterations take place in the gums, teeth, and lower jaw.
Madame von Chauvin[59]was able, by gradually accustoming Axolotl larvæ to breathe, artificially to cause them to undergo the above metamorphosis.
It seems very possible, as suggested by Weismann[60], that the existing Axolotls are really descendants of Amblystoma forms, which have reverted to a lower stage. In favour of this possibility a very interesting discovery of Filippi’s[61]may be cited. He found in a pond in a marsh near Andermat some examples of Triton alpestris, which, though they had become sexually mature, still retained the external gills and the other larval characters. Similar sexually mature larval forms of Triton tæniatus have been described by Jullien. These discoveries would seem to indicate that it might be possible artificially to cause the Newt to revert to a perennibranchiate condition.
Gymnophiona. The development of the Gymnophiona is almost unknown, but it is certain that some larval forms are provided with a single gill-cleft, while others have external gills.
A gill-cleft has been noticed in Epicrium glutinosum (Müller), and in Cœcilia oxyura. In Cœcilia compressicauda, Peters (No.108) was unable to find any trace of a gill-cleft, but he observed in the larvæ within the uterus two elongated vesicular gills.
Bibliography.
Amphibia.
(93)Ch. van Bambeke. “Recherches sur le développement du Pélobate brun.”Mémoires couronnés, etc. de l'Acad. roy. de Belgique, 1868.(94)Ch. van Bambeke. “Recherches sur l'embryologie des Batraciens.”Bulletin de l'Acad. roy. de Belgique, 1875.(95)Ch. van Bambeke. “Nouvelles recherches sur l'embryologie des Batraciens.”Archives de Biologie,Vol.I.1880.(96)K. E. von Baer. “Die Metamorphose des Eies der Batrachier.” Müller’sArchiv, 1834.(97)B. Benecke. “Ueber die Entwicklung des Erdsalamanders.”Zoologischer Anzeiger, 1880.(98)S. F. Clarke. “Development of Amblystoma punctatum,” PartI., External.Studies from the Biological Laboratory of the Johns Hopkins University,No.II.1880.(99)H. Cramer. “Bemerkungen üb. d. Zellenleben in d. Entwick. d. Froscheies.” Müller’sArchiv, 1848.(100)A. Ecker.Icones Physiolog.1851-1859.(101)A. Götte.Die Entwicklungsgeschichte der Unke.Leipzig, 1875.(102)C. K. Hoffmann. “Amphibia.”Klassen u. Ordnungen d. Thierreichs, 1873-1879.(103)T. H. Huxley. Article “Amphibia” in theEncyclopædia Britannica.(104)A. Moquin-Tandon. “Développement des Batraciens anures.”Annales des Sciences Naturelles,III.1875.(105)G. Newport. “On the impregnation of the Ovum in Amphibia” (three memoirs).Phil. Trans.1851, 1853, and 1854.(106)W. K. Parker. “On the structure and development of the Skull of the common Frog.”Phil. Trans.,CLXI.1871.(107)W. K. Parker. “On the structure and development of the Skull of the Batrachia.”Phil. Trans.,Vol.CXLVI., Part 2. 1876.(108)W. C. H. Peters. “Ueber die Entwicklung der Coecilien und besonders von Cœcilia compressicauda.”Berlin Monatsbericht,p.40, 1874.(109)W. C. H. Peters. “Ueber die Entwicklung der Coecilien.”Berl. Monatsbericht,p.483, 1875.(110)J. L. PrevostandJ. B. Dumas. “Deuxième Mém. s. l. génération. Développement de l'œuf d. Batraciens.”Ann. Sci. Nat.II.1824.(111)R. Remak.Untersuchungen über die Entwicklung der Wirbelthiere, 1850-1858.(112)M. Rusconi.Développement de la grenouille commune depuis le moment de sa naissance jusqu'à son état parfait, 1826.(113)M. Rusconi.Histoire naturelle, développement et métamorphose de la Salamandre terrestre, 1854.(114)W. B. ScottandH. F. Osborn. “On the early development of the common Newt.”Quart. J. of Micr. Science,Vol.XXIX.1879.(115)S. Stricker. “Entwicklungsgeschichte von Bufo cinereus.”Sitzb. der kaiserl. Acad. zu Wien, 1860.(116)S. Stricker. “Untersuchungen über die ersten Anlagen in Batrachier-Eiern.”Zeitschrift f. wiss. Zoologie,Bd.XI.1861.
[42]The following classification of the Amphibia is employed in the present chapter:I. Anura.Aglossa.Phaneroglossa.II. Urodela.PerennibranchiataTrachystomata.Proteidæ.CaducibranchiataAmphiumidæ.Menopomidæ.MyctoderaAmblystomidæ.Salamandridæ.III. Gymnophiona.[43]I am under great obligations to Mr Parker for having kindly supplied me, in answer to my questions, with a large amount of valuable information on the development of the Amphibia.[44]Within the vitelline membrane there appears to be present, in the Anura at any rate, a very delicate membrane closely applied to the yolk.[45]Allen Thomson informs me that he has watched the process of fertilization in the Newt, and that the male deposits the semen in the water close to the female. From the water it seems to enter the female generative aperture. Von Siebold has shewn that there is present in female Newts and Salamanders a spermatic bursa. In this bursa the spermatozoa long (three months) retain their vitality in some Salamanders. Various peculiarities in the gestation are to be explained by this fact.[46]For a summary of these and the literature of the subjectvide“Amphibia,” by C. K. Hoffmann, in Bronn’sClassen und Ordnungen d. Thier-reichs.[47]VideSpengel,“Die Fortpflanzung des Rhinoderma Darwinii.”Zeit. f. wiss. Zool.,Bd.XXIX.,1877. This paper contains a translation of a note byJiminez de la Espadaon the development of the species.[48]Since the body cavity is not developed as diverticula from the cavity of invagination, the latter cavity may conveniently be called the mesenteron and not the archenteron.[49]VideHuxley, “Craniofacial apparatus of Petromyzon.”Journ. of Anat. and Phys.Vol.X.1876. Huxley’s views about the Meckelian arch, etc., are plausible, but it seems probable from Scott’s observations that true branchial bars are not developed in the Lamprey. How far this fact necessarily disproves Huxley’s views is still doubtful.[50]Conf. Huxley and Götte.[51]Cf.Parker (No.107).[52]Annal. de Sciences Nat., 5th Series,Vol.XVII., 1873.[53]Berlin. Monatsbericht, 1876,p.703, andNature, April 5, 1877.[54]Proceed. of Boston Nat. Hist. Society,Vol.V., 1854.[55]In confirmation of this view it may be mentioned that in Pipa Americana the tail appears to function as a respiratory organ in the later stages of development (Peters).[56]The recent observations on this subject are those of Scott and Osborn (No.114) on Triton, of Bambeke (No.95) on various species of Triton and the Axolotl, and of Clark (No.98) on Amblystoma punctatum.[57]Allen Thomson informs me that the crested Newt, Triton cristatus, is in rare instances viviparous.[58]Comptes Rendus, 1870,p.782.[59]Zeit. f. wiss. Zool.,Bd.XXVII. 1876.[60]Zeit. f. wiss. Zool.,Bd.XXV. sup. 1875.[61]Archivio per la Zoologia, l'Anatomia e la Fisiologia,Vol. 1. Genoa, 1861. Conf. alsoVon Siebold, “Ueber die geschlechtliche Entwicklung d. Urodelen-Larven.”Zeit. f. wiss. Zool.,Bd.XXVIII., 1877.
[42]The following classification of the Amphibia is employed in the present chapter:
I. Anura.
Aglossa.
Phaneroglossa.
II. Urodela.
Perennibranchiata
Trachystomata.
Proteidæ.
Caducibranchiata
Amphiumidæ.
Menopomidæ.
Myctodera
Amblystomidæ.
Salamandridæ.
III. Gymnophiona.
[43]I am under great obligations to Mr Parker for having kindly supplied me, in answer to my questions, with a large amount of valuable information on the development of the Amphibia.
[44]Within the vitelline membrane there appears to be present, in the Anura at any rate, a very delicate membrane closely applied to the yolk.
[45]Allen Thomson informs me that he has watched the process of fertilization in the Newt, and that the male deposits the semen in the water close to the female. From the water it seems to enter the female generative aperture. Von Siebold has shewn that there is present in female Newts and Salamanders a spermatic bursa. In this bursa the spermatozoa long (three months) retain their vitality in some Salamanders. Various peculiarities in the gestation are to be explained by this fact.
[46]For a summary of these and the literature of the subjectvide“Amphibia,” by C. K. Hoffmann, in Bronn’sClassen und Ordnungen d. Thier-reichs.
[47]VideSpengel,“Die Fortpflanzung des Rhinoderma Darwinii.”Zeit. f. wiss. Zool.,Bd.XXIX.,1877. This paper contains a translation of a note byJiminez de la Espadaon the development of the species.
[48]Since the body cavity is not developed as diverticula from the cavity of invagination, the latter cavity may conveniently be called the mesenteron and not the archenteron.
[49]VideHuxley, “Craniofacial apparatus of Petromyzon.”Journ. of Anat. and Phys.Vol.X.1876. Huxley’s views about the Meckelian arch, etc., are plausible, but it seems probable from Scott’s observations that true branchial bars are not developed in the Lamprey. How far this fact necessarily disproves Huxley’s views is still doubtful.
[50]Conf. Huxley and Götte.
[51]Cf.Parker (No.107).
[52]Annal. de Sciences Nat., 5th Series,Vol.XVII., 1873.
[53]Berlin. Monatsbericht, 1876,p.703, andNature, April 5, 1877.
[54]Proceed. of Boston Nat. Hist. Society,Vol.V., 1854.
[55]In confirmation of this view it may be mentioned that in Pipa Americana the tail appears to function as a respiratory organ in the later stages of development (Peters).
[56]The recent observations on this subject are those of Scott and Osborn (No.114) on Triton, of Bambeke (No.95) on various species of Triton and the Axolotl, and of Clark (No.98) on Amblystoma punctatum.
[57]Allen Thomson informs me that the crested Newt, Triton cristatus, is in rare instances viviparous.
[58]Comptes Rendus, 1870,p.782.
[59]Zeit. f. wiss. Zool.,Bd.XXVII. 1876.
[60]Zeit. f. wiss. Zool.,Bd.XXV. sup. 1875.
[61]Archivio per la Zoologia, l'Anatomia e la Fisiologia,Vol. 1. Genoa, 1861. Conf. alsoVon Siebold, “Ueber die geschlechtliche Entwicklung d. Urodelen-Larven.”Zeit. f. wiss. Zool.,Bd.XXVIII., 1877.
Introduction.
The variations in the character of the embryonic development of the Amniota are far less important than in the case of the Ichthyopsida. There are, it is true, some very special features in the early developmental history of the Mammalia, but apart from these there is such a striking uniformity in the embryos of all the groups that it would, in many cases, be difficult to assign a young embryo to its proper class.
Amongst the Sauropsida the Aves have for obvious reasons received a far fuller share of attention than any other group; and an account of their embryology forms a suitable introduction to this part of our subject. For the convenience of the student many parts of their developmental history will be dealt with at greater length than in the case of the previous groups.
The development of the Aves.
Comparatively few types of Birds have been studied embryologically. The common Fowl has received a disproportionately large share of attention; although within quite recent times theDuck, the Goose, the Pigeon, the Starling, and a Parrot (Melopsittacus undulatus) have also been studied. The result of these investigations has been to shew that the variations in the early development of different Birds are comparatively unimportant. In the sequel the common Fowl will be employed as type, attention being called when necessary to the development of the other forms.
Illustration: Figure 85Fig. 85. Yolk elements from the egg of the Fowl.A.Yellow yolk.B.White yolk.
Fig. 85. Yolk elements from the egg of the Fowl.A.Yellow yolk.B.White yolk.
The ovum of the Fowl, at the time when it is clasped by the expanded extremity of the oviduct, is a large yellow body enclosed in a vitelline membrane. It is mainly formed of spherules of food-yolk. Of these there are two varieties; one known as yellow yolk, and the other as white. The white yolk spherules form a small mass at the centre of the ovum, which is continued to the surface by a narrow stalk, and there expands into a somewhat funnel-shaped disc, the edges of which are continued over the surface of the ovum as a delicate layer. The major part of the ovum is formed of yellow yolk. The yellow yolk consists of large delicate spheres, filled with small granules (fig. 85A); while the white yolk is formed of vesicles of a smaller size than the yellow yolk spheres, in which are a variable number of highly refractive bodies (fig. 85B).
Illustration: Figure 86Fig. 86. Section through the germinal disc of the ripe ovarian ovum of a Fowl while yet enclosed in its capsule.a.Connective-tissue capsule of the ovum;b.epithelium of the capsule, at the surface of which nearest the ovum lies the vitelline membrane;c.granular material of the germinal disc, which becomes converted into the blastoderm. (This is not very well represented in the woodcut. In sections which have been hardened in chromic acid it consists of fine granules.)w.y.white yolk, which passes insensibly into the fine granular material of the disc;x.germinal vesicle enclosed in a distinct membrane, but shrivelled up;y.space originally completely filled up by the germinal vesicle, before the latter was shrivelled up.
Fig. 86. Section through the germinal disc of the ripe ovarian ovum of a Fowl while yet enclosed in its capsule.a.Connective-tissue capsule of the ovum;b.epithelium of the capsule, at the surface of which nearest the ovum lies the vitelline membrane;c.granular material of the germinal disc, which becomes converted into the blastoderm. (This is not very well represented in the woodcut. In sections which have been hardened in chromic acid it consists of fine granules.)w.y.white yolk, which passes insensibly into the fine granular material of the disc;x.germinal vesicle enclosed in a distinct membrane, but shrivelled up;y.space originally completely filled up by the germinal vesicle, before the latter was shrivelled up.
In addition to the yolk there is present in the ovum a small protoplasmic region, containing the remains of the germinal vesicle, which forms the germinal disc (fig. 86). It overlies thefunnel-shaped disc of white yolk, into which it is continued without any marked line of demarcation. It contains numerous minute spherules of the same nature as the smallest white yolk spherules.
Impregnation takes place at the upper extremity of the oviduct.
In its passage outwards the ovum gradually receives its accessory coverings in the form of albumen, shell-membrane, and shell (fig. 87).
Illustration: Figure 87Fig. 87. Diagrammatic section of an unincubated Fowl’s egg.bl.blastoderm;w.y.white yolk. This consists of a central flask-shaped mass and a number of layers concentrically arranged around it.y.y.yellow yolk;v.t.vitelline membrane;x.layer of more fluid albumen immediately surrounding the yolk;w.albumen consisting of alternate denser and more fluid layers;ch.l.chalaza;a.ch.air-chamber at the broad end of the egg. This chamber is merely a space left between the two layers of the shell-membrane.i.s.m.internal layer of shell-membrane;s.m.external layer of shell-membrane;s.shell.
Fig. 87. Diagrammatic section of an unincubated Fowl’s egg.bl.blastoderm;w.y.white yolk. This consists of a central flask-shaped mass and a number of layers concentrically arranged around it.y.y.yellow yolk;v.t.vitelline membrane;x.layer of more fluid albumen immediately surrounding the yolk;w.albumen consisting of alternate denser and more fluid layers;ch.l.chalaza;a.ch.air-chamber at the broad end of the egg. This chamber is merely a space left between the two layers of the shell-membrane.i.s.m.internal layer of shell-membrane;s.m.external layer of shell-membrane;s.shell.
Illustration: Figure 88Fig 88. Surface views of the early stages of the segmentation in a Fowl’s egg.(After Coste.)a.edge of germinal disc;b.vertical furrow;c.small central segment;d.larger peripheral segment.
Fig 88. Surface views of the early stages of the segmentation in a Fowl’s egg.(After Coste.)a.edge of germinal disc;b.vertical furrow;c.small central segment;d.larger peripheral segment.
Illustration: Figure 89Fig. 89. Surface view of the germinal disc of Fowl’s egg during a late stage of the segmentation.c.small central segmentation spheres;b.larger segments outside these;a.large, imperfectly circumscribed, marginal segments;e.margin of germinal disc.
Fig. 89. Surface view of the germinal disc of Fowl’s egg during a late stage of the segmentation.c.small central segmentation spheres;b.larger segments outside these;a.large, imperfectly circumscribed, marginal segments;e.margin of germinal disc.
The segmentation commences in the lower part of the oviduct, shortly before the shell has begun to be formed. It is meroblastic, being confined to the germinal disc, through the full depth of which however the earlier furrows do not extend. It is mainly remarkable for being constantly somewhat unsymmetrical (Kölliker)—a feature which is not represented infig. 88, copied from Coste. Owing to the absence of symmetry the cells at one side of the germinal disc are larger than those at the other, but the relations between the disc and the axis of theembryo are not known. During the later stages the segmentation is irregular, and not confined to the surface; and towards its close the germinal disc becomes somewhat lenticular in shape; and is formed of segments, which are smallest in the centre and increase in size towards the periphery (figs.89and90). The superficial segments in the centre of the germinal disc are moreover smaller than those below, and more or less separated as a distinct layer (fig. 90). As development proceeds the segmentation reaches its limits in the centre, but continues at the periphery; and thus eventually the masses at the periphery become of the same size as those at the centre. At the time when the ovum is laid (fig. 91) the uppermost layer of segments has given rise to a distinct membrane, the epiblast, formed of a single row of columnarcells (ep). The lower or hypoblast segments are larger, in some cases very much larger, than those of the epiblast, and are so granular that their nuclei can only with difficulty be seen. They form a somewhat irregular mass, several layers deep, and thicker at the periphery than at the centre: they rest on a bed of white yolk, from which they are in parts separated by a more or less developed cavity, which is probably filled with fluid yolk matter about to be absorbed. In the bed of white yolk nuclei are present, which are of the same character, and have the same general fate, as those in Elasmobranchii. They are generally more numerous in the neighbourhood of the thickened periphery of the blastoderm than elsewhere. Peculiar large spherical bodies are to be found amongst the lower layer cells, which superficially resemble the larger cells around them, and have been called formative cells [videFoster and Balfour (No.126)]. Their real nature is still very doubtful, and though some are no doubt true cells, others are perhaps only nutritive masses of yolk. In a surface view the blastoderm, as the segmented germinal disc maynow be called, appears as a circular disc; the central part of which is distinguished from the peripheral by its greater transparency, and forms what is known in the later stages as the area pellucida. The narrow darker ring of blastoderm, outside the area pellucida, is the commencing area opaca.
Illustration: Figure 90Fig. 90. Section of the germinal disc of a Fowl during the later stages of segmentation.The section, which represents rather more than half the breadth of the blastoderm (the middle line being shewn atc), shews that the upper and central parts of the disc segment faster than those below and towards the periphery. At the periphery the segments are still very large. One of the larger segments is shewn ata. In the majority of segments a nucleus can be seen; and it seems probable that the nucleus is present in them all. Most of the segments are filled with highly refracting spherules, but these are more numerous in some cells (especially the larger cells near the yolk) than in others. In the central part of the blastoderm the upper cells have commenced to form a distinct layer. No segmentation cavity is present.a.large peripheral cell;b.larger cells of the lower parts of the blastoderm;c.middle line of blastoderm;e.edge of the blastoderm adjoining the white yolk;w.white yolk.
Fig. 90. Section of the germinal disc of a Fowl during the later stages of segmentation.The section, which represents rather more than half the breadth of the blastoderm (the middle line being shewn atc), shews that the upper and central parts of the disc segment faster than those below and towards the periphery. At the periphery the segments are still very large. One of the larger segments is shewn ata. In the majority of segments a nucleus can be seen; and it seems probable that the nucleus is present in them all. Most of the segments are filled with highly refracting spherules, but these are more numerous in some cells (especially the larger cells near the yolk) than in others. In the central part of the blastoderm the upper cells have commenced to form a distinct layer. No segmentation cavity is present.a.large peripheral cell;b.larger cells of the lower parts of the blastoderm;c.middle line of blastoderm;e.edge of the blastoderm adjoining the white yolk;w.white yolk.
Illustration: Figure 91Fig. 91. Section of a blastoderm of a Fowl’s egg at the commencement of incubation.The thin epiblastepcomposed of columnar cells rests on the incomplete lower layerl, composed of larger and more granular hypoblast cells. The lower layer is thicker in some places than in others, and is especially thick at the periphery. The line below the under layer marks the upper surface of the white yolk. The larger so-called formative cells are seen atb, lying on the white yolk. The figure does not take in quite the whole breadth of the blastoderm; but the reader must understand that both to the right hand and to the leftepis continued farther thanl, so that at the extreme edge it rests directly on the white yolk.
Fig. 91. Section of a blastoderm of a Fowl’s egg at the commencement of incubation.The thin epiblastepcomposed of columnar cells rests on the incomplete lower layerl, composed of larger and more granular hypoblast cells. The lower layer is thicker in some places than in others, and is especially thick at the periphery. The line below the under layer marks the upper surface of the white yolk. The larger so-called formative cells are seen atb, lying on the white yolk. The figure does not take in quite the whole breadth of the blastoderm; but the reader must understand that both to the right hand and to the leftepis continued farther thanl, so that at the extreme edge it rests directly on the white yolk.
As a result of incubation the blastoderm undergoes a series of changes, which end in the definite formation of three germinal layers, and in the establishment of the chief systems of organs of the embryo. The more important of these changes are accomplished in the case of the common Fowl during the first day and the early part of the second day of incubation.
There is hardly any question in development which has been the subject of so much controversy as the mode of formation of the germinal layers in the common Fowl. The differences in the views of authors have been caused to a large extent by the difficulties of the investigation, but perhaps still more by the fact that many of the observations were made at a time when the methods of making sections were very inferior to those of the present day. The subject itself is by no means of an importance commensurate with the attention it has received. The characters which belong to the formation of the layers in the Sauropsida are secondarily derived from those in the Ichthyopsida, and are of but little importance for the general questions which concern the nature and origin of the germinal layers. In the account in the sequel I have avoided as much as possible discussion of controverted points. My statements are founded in the main on my own observations, more especially on a recent investigation carried on in conjunction with my pupil, Mr Deighton. It is to Kölliker (No.135), and to Gasser (No.127) that the most important of the more recent advances in our knowledge are due. Kölliker,in his great work on Embryology, definitely established the essential connection between the primitive streak and the formation of the mesoblast; but while confirming his statement on this head, I am obliged to differ from him with reference to some other points.
Gasser’s work, especially that part of it which relates to the passages leading from the neural to the alimentary canal, which he was the first to discover, is very valuable.
The blastoderm gradually grows in size, and extends itself over the yolk; the growth over the yolk being very largely effected by an increase in the size of the area opaca, which during this process becomes more distinctly marked off from the area pellucida. The area pellucida gradually assumes an oval form, and at the same time becomes divided into a posterior opaque region and an anterior transparent region. The posterior opacity is named by some authors the embryonic shield.
Illustration: Figure 92Fig. 92. Transverse section through the blastoderm of a Chick before the appearance of the primitive streak.The epiblast is represented somewhat diagrammatically. The hyphens shew the points of junction of the two halves of the section.
Fig. 92. Transverse section through the blastoderm of a Chick before the appearance of the primitive streak.The epiblast is represented somewhat diagrammatically. The hyphens shew the points of junction of the two halves of the section.
During these changes the epiblast (fig. 92) becomes two layers deep over the greater part of the area pellucida, though still only one cell deep in the area opaca. The irregular hypoblast spheres of the unincubated blastoderm flatten themselves out, and unite into a definite hypoblastic membrane (fig. 92). Between this membrane and the epiblast there remain a number of scattered cells (fig. 92) which cannot however be said to form a definite layer altogether distinct from the hypoblast. They are almost entirely confined to the posterior part of the area pellucida, and give rise to the opacity of that part.
At the edge of the area pellucida the hypoblast becomes continuous with a thickened rim of material, underlying the epiblast, and derived from the original thickened edge of the blastoderm and the subjacent yolk. It is mainly formed of yolk granules,with a varying number of cells and nuclei imbedded in it. It is known as the germinal wall, and is spoken of more in detail onpp.160and161.
Illustration: Figure 93Fig. 93. Diagrams illustrating the position of the blastopore, and the relation of the embryo to the yolk in various meroblastic vertebrate ova.A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate.mg.medullary plate;ne.neurenteric canal;bl.portion of blastopore adjoining the neurenteric canal. In B this part of the blastopore is formed by the edges of the blastoderm meeting and forming a linear streak behind the embryo; and in C it forms the structure known as the primitive streak.yk.part of yolk not yet enclosed by the blastoderm.
Fig. 93. Diagrams illustrating the position of the blastopore, and the relation of the embryo to the yolk in various meroblastic vertebrate ova.A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate.mg.medullary plate;ne.neurenteric canal;bl.portion of blastopore adjoining the neurenteric canal. In B this part of the blastopore is formed by the edges of the blastoderm meeting and forming a linear streak behind the embryo; and in C it forms the structure known as the primitive streak.yk.part of yolk not yet enclosed by the blastoderm.
The changes which next take place result in the complete differentiation of the embryonic layers, a process which is intimately connected with the formation of the structure known as theprimitive streak. The meaning of the latter structure, and its relation to the embryo, can only be understood by comparison with the development of the forms already considered. The most striking peculiarity in the first formation of the embryo Bird, as also in that of the embryos of all Amniota, consists in the fact thatthey do not occupy a position at the edgeof the blastoderm, but are placed near its centre. Behind the embryo there is however a peculiar structure—the primitive streak above mentioned—which is a linear body placed in the posterior region of the blastoderm. This body, the nature of which will be more fully explained in the chapter on the comparative development of Vertebrates, is really a rudimentary part of the blastopore, of the same nature as the linear streak behind the embryo in Elasmobranchii formed by the concrescence of the edges of the blastoderm (videp. 64); although there is no ontogenetic process in the Amniota, like the concrescence in Elasmobranchii. The relations of the blastopore in Elasmobranchii and Aves is shewn infigs.B and C of the diagram (fig. 93).
Illustration: Figure 94Fig. 94. Area pellucida of a very young blastoderm of a chick, shewing the primitive streak at its first appearance.pr.s.primitive streak;ap.area pellucida;a.op.area opaca.
Fig. 94. Area pellucida of a very young blastoderm of a chick, shewing the primitive streak at its first appearance.pr.s.primitive streak;ap.area pellucida;a.op.area opaca.
Illustration: Figure 95Fig. 95. Transverse section through a blastoderm of about the age represented in fig. 94, shewing the first differentiation of the primitive streak.The section passes through about the middle of the primitive streak.pvs.primitive streak;ep.epiblast;hy.hypoblast;yk.yolk of the germinal wall.
Fig. 95. Transverse section through a blastoderm of about the age represented in fig. 94, shewing the first differentiation of the primitive streak.The section passes through about the middle of the primitive streak.pvs.primitive streak;ep.epiblast;hy.hypoblast;yk.yolk of the germinal wall.
In describing in detail the succeeding changes we may at first confine our attention to the area pellucida. As this gradually assumes an oval form the posterior opacity becomes replaced by a very dark median streak, which extends forwards some distance from the posterior border of the area (fig. 94). This is the first rudiment of the primitive streak. In theregion in front of it the blastoderm is still formed of two layers only, but in the region of the streak itself the structure of the blastoderm is greatly altered. The most important features in it are represented infig. 95. This figure shews that the median portion of the blastoderm has become very much thickened (thus producing the opacity of the primitive streak), and that this thickening is caused by a proliferation of rounded cells from the epiblast. In the very young primitive streak, of whichfig. 95is a section, the rounded cells are still continuous throughout with the epiblast, but they form nevertheless the rudiment of the greater part of a sheet of mesoblast, which will soon arise in this region.
In addition to the cells clearly derived from the epiblast, there are certain other cells (videfig. 95), closely adjoining the hypoblast, which appear to me to be the derivatives of the cells interposed between the epiblast and hypoblast, which gave rise to the posterior opacity in the blastoderm during the previous stage. In my opinion these cells also have a share in forming the future mesoblast.
The number and distribution of these cells is subject to not inconsiderable variations. In a fair number of cases they are entirely congregated along the line of the primitive streak, leaving the sides of the blastoderm quite free. They then form a layer, which can only with difficulty be distinguished from the cells derived from the epiblast by slight peculiarities of staining, and by the presence of a considerable proportion of large granular cells. It is, I believe, by the study of such blastoderms that Kölliker has been led to deny to the intermediate cells of the previous stage any share in the formation of the mesoblast. In other instances, of whichfig. 95is a fairly typical example, they are more widely scattered. To follow with absolute certainty the history of these cells, and to prove that they join the mesoblast is not, I believe, possible by means of sections, and I must leave the reader to judge how far the evidence given in the sequel is sufficient to justify my opinions on this subject.