86 DEVELOPMENT OF THE MAMMALIAN EMBRYO. 



vesicle in the embryos of all Mammalia yet examined, on its being 

 invariably situated on the outside of the body of the embryo, and on its 

 disappearance in all, either before, or at the termination of intra-uterine life, 

 Bischoff * confirms the observations of Von Baer and Coste,f that in its 

 ulterior relations to the embryo this vesicle presents considerable varieties 

 in the different classes^of Mammalia. In ruminants and pachyderms, 

 although at its first formation the umbilical vesicle grows with ex- 

 treme rapidity, yet very shortly its development is arrested, and it then 

 begins to disappear. In the embryos of cows not more than six lines 

 long, the middle portion alone is found still in existence, and its attach- 

 ment to the intestine consists no longer of a canal, but only of a thread- 

 like pedicle. Its blood-vessels also have undergone a proportionate 

 reduction. In the pig an almost equally early disappearance of the 

 vesicle is observed. In Carnivora, on the contrary, the umbilical vesicle 

 remains during the whole period of intra-uterine existence ; presenting 

 itself as a cylindrical sac, the surface of which is throughout abundantly 

 supplied with omphalo-mesenteric vessels, and the cavity of which is long- 

 in direct communication with that of the intestine. The left side of the 

 vesicle is covered by the allantois, while its right side is in contact with 

 the chorion. At a certain period the vesicle surrounds the upper part of 

 the embryo in such a manner that this part appears as if contained 

 within its cavity, and has been so described by some embryologists. But 

 at a later period when the embryo has detached itself from this relation to 

 the vesicle, the error of such a supposition is rendered manifest. In rodents, 

 the rabbit for example, the umbilical vesicle also persists during the whole 

 of intra-uterine life, and at one period there is observed a similar deceptive 

 appearance as in Carnivora, of the embryo being contained within the 

 cavity of the vesicle. 



Allantois. According to Bischoff,J the allantois of the Mammalian 

 embryo is developed neither from the intestinal tube, as stated by Von 

 Baer, Kathke, Valentin, and others, nor from the Wolffian bodies, as was 

 said by Reichert, to be the case in the chick ; for at the time of its first 

 appearance no trace either of the intestinal canal or of the Wolffian bodies 

 can be perceived. At its earliest appearance, the allantois in the rabbit 

 consists of a solid mass of cells projecting from the visceral plate of the 

 tail. But in the dog this mass is at first double (figs. 13 and 14), though 

 the two halves soon fuse together, and are converted into a single vesicle. 

 The allantois is abundantly vascular, for it contains the ramifying ex- 

 tremities of the two arteries which run along the sides of the vertebral 

 arches, and of the two veins which are situated within the walls of the 

 visceral laminae. These vessels subsequently become the umbilical arteries 



* Op. cit.p. 113. t Mailer's Physiology, p. 1570. 



J Entwick. der Saugeth. und des Menschen, p. 116 ; and Entwick. des Kaninclien-eies, 

 p. 122. Miiller's Physiology, p. 1554. 



