in the Foetus of Vertebrateil Animals, 263 



animals, such as the sheep or cow, and also in the pig and horse, 

 the yolk ceases to increase at a very early period ; the part by 

 which it communicates with the intestine is lengthened out, and 

 the sac of the yolk, collapsed and empty, remains hanging for 

 some time from the funis of the umbilicus, attached by a long 

 and narrow cord to a projecting fold of intestine (Fig. 32, xy). 

 The yolk at this period has generally received the name of 

 Umbilical Vesicle ; it still retains a yellowish colour, a spongy 

 granular consistence, and the ramifications of vessels are visible 

 on it till it at length disappears. 



In carnivorous animals the umbilical vesicle, or sac of the yolk, 

 resembles, more than in any others that have been examined, 

 the sac of the yolk in birds. In the cat it is filled with a sub- 

 stance of a dark yellow colour. In the progress of development, 

 however, it comes to have the shape of a long narrow vesicle 

 lying parallel to the long diameter of the foetus, and fastened by 

 its two pointed extremities to the chorion, or outer membrane of 

 the ovum (Fig. 33, of the Dog). 



In the ovum of the human species,* the yolk-sac, or umbilical 

 vesicle, is very small and globular, and disappears shortly after 

 the end of the second month. But in the rodentia, as in the rabbit 

 or hare, the umbilical vesicle is highly developed, and the blood- 

 vessels distributed on it comparatively numerous and large. At 

 first this vesicle resembles much the yolk-sac of birds (Fig. 34), 

 but in the later stages of development, it is expanded so as to 

 form a vascular covering over nearly all the parts of the ovum. 



The Allantoid Membrane, or expanded portion of the urinarv 

 bladder, becomes developed in the foetus of mammalia, in the 

 same manner as in that of lizards, serpents, and birds. The 

 umbilical vessels are ramified on it, and for some time after its 

 first appearance it preserves the same relations as in these 

 animals. 



In ruminating and pachydermatous animals in which the um- 

 bilical vesicle is small, the allantois is very highly developed. 



and Emmert in Reil's Archiv., B. x. h. i Blumenbach and Carus' Comp. 



Anat — Dutrochet in vol. viii. of the M^moires de laSoc M<^d. d'Emulation ; 

 and Bojanus in Meckel's Archiv. B. iv., and in Nov. Act. Phys. Med. torn. x. 



• See the description of the membranes in the human embryo, in the 

 Traite d'Accoiichemens by Velpeau. Paris, 1829. Vol. ii. p. 2Sl). 



