The Works of Francis Balfour 3-9
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
- Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol. III. A Treatise on Comparative Embryology 2 (1885)
CHAPTER IX. REPTILIA
THE formation of the germinal layers in the Reptilia is very imperfectly known. The Lizard has been studied in this respect more completely than other types, and there are a few scattered observations on Turtles and Snakes.
The ovum has in all Reptilia a very similar structure to that in Birds. Impregnation is effected in the upper part of the oviduct, and the early stages of development invariably take place in the oviduct. A few forms are viviparous, viz. some of the blindworms amongst Lizards (Anguis, Seps), and some of the Viperidae and Hydrophidae amongst the Serpents. In the majority of cases, however, the eggs are laid in moist earth, sand, &c. Around the true ovum an egg-shell (of the same general nature as that in birds, though usually soft), and a variable quantity of albumen, are deposited in the oviduct. The extent to which development has proceeded in the oviparous forms before the eggs are laid varies greatly in different species.
The general features of the development (for a knowledge of which we are mainly indebted to Rathke's beautiful memoirs), the structure of the amnion and allantois, &c. are very much the same as in Birds.
The Lizards will be taken as type of the class, and a few noteworthy points in the development of other groups will be dealt with at the close of the Chapter. The following description, taken in the main from my own observations, applies to Lacerta muralis.
The segmentation is meroblastic, and similar to that in Birds. At its close the resulting blastoderm becomes divided into two layers, a superficial epiblast formed of a single row of cells, and
a layer below this several rows deep. Below this layer fresh segments continue for some time to be added to the blastoderm from the subjacent yolk.
The blastoderm, which is thickened at its edge, spreads rapidly over the yolk. Shortly before the yolk is half enclosed a small embryonic shield (area pellucida) makes its appearance near the centre of the blastoderm. The embryonic shield is mainly distinguished from the remainder of the blastoderm by the more columnar character of its constituent epiblast cells. It is somewhat pyriform in shape, the narrower end corresponding with the future posterior end of the embryo. At the hind end of the shield a somewhat triangular primitive streak is formed, consisting of epiblast continuous below with a great mass of rounded mesoblast cells, probably mainly formed, as in the bird, by a proliferation of the epiblast. To this mass of cells the hypoblast is also partially adherent. At the front end of the streak an epiblastic involution appears, which soon becomes extended into a passage open at both extremities, leading obliquely forwards through the epiblast to the space below the hypoblast. The walls of the passage are formed of a layer of columnar cells continuous both with epiblast and hypoblast. In front of the primitive streak the body of the embryo becomes first differentiated by the formation of a medullary plate; and at the same time there grows out from the primitive streak a layer of mesoblast,
FIG. 126. SECTIONS THROUGH AN EMBRYO OF LACERTA MURALIS REPRESENTED IN FIG. I2Q.
m.g. medullary groove ; mep. mesoblastic plate ; ep. epiblast ; hy. hypoblast ; ch' , notochordal thickening of hypoblast ; ch. notochord ; ne. neurenteric canal (blastopore). In E. ne points a diverticulum of the neurenteric canal into the primitive streak.
which spreads out in all directions between the epiblast and
204 FORMATION OF THE LAYERS.
hypoblast. In the region of the embryo the mesoblast plate is stated by Kupffer and Benecke to be continuous across the middle line, but this appears very improbable. In a slightly later stage the medullary plate becomes marked by a shallow groove, and the mesoblast of the embryo is then undoubtedly constituted of two lateral plates, one on each side of the median line. In the median line the notochord arises as a ridge-like thickening of the hypoblast, which is continued posteriorly into the front wall of the passage mentioned above.
The notochord does not long remain attached to the hypoblast, and the separation between the two is already effected for the greater part of the length of the embryo by the stage represented in fig. 129. Fig. 126 represents a series of sections through this embryo.
In a section (A) through the trunk of the embryo a short way in front of the primitive streak, there is a medullary plate with a shallow groove (mg), well-developed mesoblastic plates (mep), already divided into somatic and splanchnic layers, and a completely formed notochord independent of the hypoblast (fiy). In the next section (B), taken just in front of the primitive streak, the notochord is attached to the hypoblast, and the medullary groove is deeper ; while in the section following (C), which passes through the front border of the primitive streak,
FIG. 127. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA. //. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypoblast ; ep. epiblast of the medullary plate ; pr. primitive streak. In the primitive streak all the layers are partially fused.
the notochord and hypoblast have become fused with the epiblast. The section behind (D) shews the neurenteric passage leading through the floor of the medullary groove and through the hypoblast (ne). On the right side the mesoblastic plate has become continuous with the walls of the passage. The last section (E) passes through the front part of the primitive streak
behind the passage. The mesoblast, epiblast, and to some extent the hypoblast, are now fused together in the axial line, and in the middle of the fused mass is seen a narrow diverticulum (tie) which is probably equivalent to the posterior diverticulum of the neural canal in Birds (vide p. 164).
The general features of the stage will best be understood by an examination of the diagrammatic longitudinal section represented in fig. 127. In front is shewn the amnion (am), growing over the head of the embryo. The notochord (ch) is seen as an independent cord for the greater part of the length of the embryo, but falls into the hypoblast shortly in front of the neurenteric passage. The neurenteric passage is shewn at ne, and behind it is the front part of the primitive streak.
It is interesting to notice the remarkable relations of the notochord to the walls of the neurenteric passage. More or less similar relations are also well marked in the case of the goose and the fowl, and support the conclusion, deducible from the lower forms of Vertebrata, that the notochord is essentially hypoblastic.
The passage at the front end of the primitive streak forms the posterior boundary of the medullary plate, though the medullary groove is not at first continued back to it. The anterior wall of this passage connects together the medullary plate and the notochordal ridge of the hypoblast. In the stage represented in fig. 126 and 129 the medullary groove has become continued back to the opening of the passage, which thus becomes enclosed in the medullary folds, and forms a true neurenteric passage 1 .
It will be convenient at this point to say a few words as to what is known of the further fate of the neurenteric canal, and the early development of the allantois. According to Strahl, who has worked on Lacerta vivipara, the canal gradually closes from below upwards, and is obliterated
1 Kupffer and Benecke (No. 154) give a very different account from the above of the early Lacertilian development, more especially in what concerns the so-called neurenteric passage. They believe this structure to be closed below, and to form therefore a blind sack open externally. The open end of this sack they regard as the blastopore an interpretation which accords with my own, but they regard the sack as the rudiment of the allantois, and hold that it is equivalent to the invaginated archenteron of Amphioxus. I need scarcely say that I believe Kupffer and Benecke to have made a mistake in denying the existence of the ventral opening of this organ. Kupffer in a subsequent paper (No. 155) states that my descriptions of the structure of this organ do not correspond with the fact. I have perfect confidence in leaving the decision of this point to future observers, and may say that my observations have already been fully confirmed by Strahl (No. 160), who has also added some observations on the later stages to which I shall hereafter have occasion to allude.
before the completion of the neural canal. The hind end of the alimentary tract appears also to become a closed canal before this stage.
In Lacerta muralis the history appears to be somewhat different, and it is more especially to be noticed that in this species the hindgut does not become closed till considerably after the completion of the neural canal. In a stage shortly after that last described, the neurenteric passage becomes narrower. The next stage which I have observed is considerably
FlG. 128. FOUR TRANSVERSE SECTIONS THROUGH THE HINDER END OF A YOUNG EMBRYO OF LACERTA MURALIS.
Sections A and B pass through the whole embryo, while C and D only pass through the allantois, which at this stage projects backwards into the section of the body cavity behind the primitive streak.
ne. neurenteric canal ; pr. primitive streak ; kg. hind-gut ; hy. hypoblast ; //. body cavity; am. amnion ; se. serous envelope (outer limb of the amnion fold not yet separated from the inner limb or true amnion); al. allantois; me. mesoblastic wall of the allantois; v. vessels passing to the allantois.
later. The neural canal has become completely closed, and the flexure of the embryo has already made its appearance. There is still a well-developed, though somewhat slit-like, neurenteric passage, but from the analogy of birds, it is not impossible that it may have in the meantime closed up
and opened again. It has, in any case, the same relations as in the previous stage.
It leads from the end of the medullary canal (at the point where its walls are continuous with the cells of the primitive streak) round the end of the notochord, which here becomes continuous with the medullary cord, and so through the hypoblast. The latter layer is still a flat sheet without any lateral infolding ; but it gives rise, behind the neurenteric passage, to a blind posteriorly directed diverticulum, placed in the body cavity behind the embryo, and opening at the ventral face of the apparent hind end of the primitive streak. There is very little doubt that this diverticulum is the commencing allantois.
At a somewhat later stage the arrangement of these parts has undergone some changes. Their relations are shewn in the sections represented in fig. 128.
The foremost section (A) passes through the alimentary opening of the neurenteric passage (ne). Above this opening the section passes through the primitive streak (pr) close to its junction with the walls of the medullary canal. The hypoblast is folded in laterally, but the gut is still open below. The amnion is completely established. In the next section figured (B), the fourth of my series, the gut is completely closed in ; and the mesoblast has united laterally with the axial tissue of the primitive streak. Vessels to supply the allantois are shewn at v.
The three following sections are not figured, but they present the same features as B, except that the primitive streak gets rapidly smaller, and the lumen of the gut narrower. The section following (C) represents, I believe, only the stalk of the allantoic diverticulum; This diverticulum appears to be formed as usual of hypoblast (hy) enveloped by splanchnic mesoblast (me), and projects into the section of the body cavity present behind the embryo. Its position in the body cavity is the cause of its somewhat peculiar appearance in the figure. Had the whole section been represented the allantois would have been enclosed in a space between the serous membrane (se) and a layer of splanchnic mesoblast below which has also been omitted in fig. B 1 . It still points directly backwards, as it primitively does in the chick, vide fig. 123 A, and Gasser, No. 127, PI. v. figs, i and 2. I do not understand the apparently double character of the lumen of the allantois. In the next section (not figured) the lumen of the allantoic stalk is larger, but still apparently double, while in the last section (D) the lumen is considerably enlarged and single. The neurenteric canal appears to close shortly after the stage last described, though its further history has not been followed in detail.
1 Owing to the difficulty of procuring material I have only been able to prepare the two sets of sections just described, and in the absence of a fuller series there are some points in the interpretation of the sections which must remain doubtful.
FIG. 129. SURFACE VIEW
OF A YOUNG EMBRYO OF LACERTA MURALIS.
am. amnion streak.
General development of the Embryo.
The formation of the embryo commences with the appearance of the medullary plate, the sides of which soon grow up to form the
medullary folds. The medullarygroove \jj$^a.m
is developed anteriorly before any trace of it is visible behind. In a general way the closure of the groove takes place as in Birds, but the anterior part of the body is very early folded off, sinks into the yolk, and becomes covered over by the amnion as by a hood (figs. 127 and 129). All this takes place before the closure of the medullary canal ; and the changes of this part are quite concealed from view.
The closure of the medullary canal commences in the neck, and extends forwards and backwards ; and the whole region of the brain becomes closed in, while the groove is still largely open behind.
The later stages in the development of the Lacertilian embryo do not require a detailed description, as they present the closest analogy with those already described for Aves. The embryo soon turns on to its left side ; and then, becoming continuously folded off from the yolk, passes through the series of changes of form with which the reader is already familiar. An advanced embryo is represented in fig. 130. The early development and great length of the tail, which is spirally coiled on the ventral surface, is a special feature to which the attention of the reader may be called.
Embryonic Membranes and Yolk-Sack.
The early development of the cephalic portion of the amnion has already been alluded to. The first traces of it become apparent while the medullary groove is still extremely shallow. The medullary plate in the region of the head forms an axial strip of a thickish plate of epiblast. The edge of this plate
coincides with the line of the amniotic fold, and as this fold rises up the two sides of the plate become bent over the embryo and give rise to the inner limb of the amnion or amnion proper. The section (fig. 127), representing the origin of the amniotic hood of the head, shews very well how the space between the two limbs of the amnion is continuous with the body cavity. The amnion very early completely encloses the embryo (fig. 128 A and B), and its external limb or serous membrane, after separating from the true amnion, soon approaches and fuses with the vitelline membrane.
The first development of the allantois as a diverticulum of the hypoblast covered by splanchnic mesoblast, at the apparent posterior end of the primitive streak, has been described on p. 207. The allantois continues for some time to point directly backwards; but
gradually assumes a ^\^^^ , j>6
more ventral direction ; and, as it increases in size, extends into the space between the serous membrane and amnion, eventually to form a large, highly vascular, flattened sack immediately below the serous membrane.
The Yolk - Sack. The blastoderm spreads in the Lizard with very great rapidity over the yolk to form the yolksack. The early appearance of the area pellucida, or as it has been called by Kupffer and Benecke the embryonic shield, has already been noted. Outside this a vascular area, which has the same function as
FIG. 130. ADVANCED EMBRYO OF LACERTA MURALIS AS AN OPAQUE OBJECT 1 .
The embryo was 7 mm. in length in the curled up state.
fb. fore-brain ; mb. mid-brain ; cb. cerebellum ; au. auditory vesicle (closed) ; ol. olfactory pit ; md. mandible ; hy. hyoid arch ; br. branchial arches ; //. fore-limb ; hi. hind-limb.
1 This figure was drawn for me by Professor Haddon. B. III. 14
in the chick, is not long in making its appearance. In all Reptilia the vascular channels which arise in the vascular area, and the vessels carrying the blood to and from the vascular area, are very similar to those in the chick. In the Snake the sinus terminalis never attains so conspicuous a development and in Chelonia the stage with a pair of vitelline arteries is preceded by a stage in which the vascular area is supplied, as it permanently is in many Mammals, by numerous transverse arterial trunks, coming off from the dorsal aorta (Agassiz, No. 164). The vascular area gradually envelops the whole yolk, although it does so considerably more slowly than the general blastoderm.
Ophidia. There is, as might have been anticipated, a very close correspondence in general development between the Lacertilia and Ophidia. The embryos of all the Amniota are, during part of their development, more or less spirally coiled about their long axis. This is well marked in the chick of the third day; it is still more pronounced in the Lizard (fig. 130) ; but it reaches its maximum in the Snake. The whole Snake embryo has at the time when most coiled (Dutrochet, Rathke) somewhat the form of a Trochus. The base of the spiral is formed by the head, while the majority of the coils are supplied by the tail. There are in all at this stage seven coils, and the spiral is right-handed.
Another point, which deserves notice in the Snake, is the absence in the embryo of all external trace of the limbs. It might have been anticipated, on the analogy of the branchial arches, that rudiments of the limbs would be preserved in the embryo even when limbs were absent in the adult. Such, however, is not the case. It is however very possible that rudiments of the branchial arches and clefts have been preserved because these structures were functional in the larva (Amphibia) after they ceased to have any importance in the adult ; and that the limbs have disappeared even in the embryo because in the course of their gradual atrophy there was no advantage to the organism in their being specially preserved at any period of life 1 .
Chelonia 2 . In their early development the Chelonia re 1 It is very probable that in those Ophidia in which traces of limbs are still preserved, that more conspicuous traces would be found in the embryos than in the adults.
- Vide Agassiz (No. 164), Kupffer and Benecke (No. 154), and Parker (No. 165).
semble, so far as is known, the Lacertilia. The amnion arises early, and soon forms a great cephalic hood. Before development has proceeded very far the embryo turns over on to its left side. The tail in many species attains a very considerable
FIG. 131. CHELONE MIDAS, FIRST STAGE.
Au. auditory capsule; br, i and 2, branchial arches; C. carapace; E. eye;f.b. fore-brain; /./. fore-limb; H. heart; h.b. hind-brain; h.l. hind-limb; hy. hyoid; m.b. mid-brain; mn. mandible; mx.p. maxillo-palatine ; N. nostril; u. umbilicus.
FIG. 132. CHELONE MIDAS, SECOND STAGE. Letters as in fig. 131.
development (fig. 133). The chief peculiarity in the form of the embryo (figs. 131, 132, and 133) is caused by the development of the carapace. The first rudiment of the carapace appears in the form of two longitudinal folds, extending above the line of insertion of the fore- and hind-limbs, which have already made their appearance (fig. 131). These folds are subsequently prolonged so as to mark out the area of the carapace on the dorsal surface. On the surface of this area there are formed the horny plates (tortoise shell), and in the mesoblast below the bony elements of the carapace (figs. 132 and 133).
FIG. 133. CHELONE MIDAS, THIRD STAGE. Letters as in fig. 131. r. rostrum.
Immediately after hatching the yolk-sack becomes withdrawn into the body ; while the external part of the allantois shrivels up.
(154) C. Kupffer and Benecke. Die erste Entwicklung am Ei d. Reptilien. Konigsberg, 1878.
(155) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere." Zoologischer Anzeiger, Vol. II. 1879, pp. 520, 593, 612.
(156) F. M. Balfour. " On the early Development of the Lacertilia, together with some observations, etc." Quart. J. of Micr. Science, Vol. XIX. 1879.
(157) Emmert u. Hochstetter. " Untersuchung Ub. d. Entwick. d. Eidechsen in ihren Eiern." Rail's Archiv, Vol. x. 1811.
(158) M. Lereboullet. " Developpement de la Truite, du Lizard et du Limnee. II. Embryologie du Lezard." An. Sci. Nat., Ser. iv., Vol. xxvn. 1862.
(159) W. K. Parker. "Structure and Devel. of the Skull in Lacertilia." Phil. Trans., Vol. 170, p. 2. 1879.
(160) H. Strahl. " Ueb. d. Canalis myeloentericus d. Eidechse." Schrift. d. Gesell. z. Bejor. d. gesam. Naturwiss. Marburg. July 23, 1880.
(161) H. Dutrochet. " Recherches s. 1. enveloppes du foetus. " Mem. d. Soc. Med. d' Emulation, Paris, Vol. vm. 1816.
(162) W. K. Parker. "On the skull of the common Snake." Phil. Trans., Vol. 169, Part II. 1878.
(163) H. Rathke. Entwick. d. Natter. Konigsberg, 1839.
(164) L. A gas si z. Contributions to the Natural History of the United States, Vol. II. 1857. Embryology of the Turtle.
(165) W. K. Parker. "On the development of the skull and nerves in the green Turtle." Proc. of the Roy. Soc., Vol. xxvm. 1879. Vide also Nature, April 14, 1879, and Challenger Reports, Vol. I. 1880.
(166) H. Rathke. Ueb. d. Entwicklung d. Schildkroten. Braunschweig, 1848.
(167) H. Rathke. Ueber die Entwicklung d. Krokodile. Braunschweig, 1866.