fusiform, and for a long time it remains connected with the 

 intestine. In human beings the yolk sac, called here the 

 umbilical sac, remains spherical. In rodents it is very large 

 and rounded, winding under the serous membrane around the 

 amnion. 



Before the beginning of the untwisting of the embryo 

 from the yolk sac, the embryo is split into two layers — 

 animal and vegetative — connected in the region of the primary 

 stripe. The abdominal layers, after separation, turn upwards. 

 Because of this the formation of the amnion quickens to the 

 extent that the moment of its formation is difficult to 

 recognize. "However," Baer wrote, "I was lucky enough to see 

 in dogs, and more frequently in swine and sheep, embryos 

 completely uncovered by the amnion. Therefore I can assure 

 absolutely that the amniotic sac is formed as that of birds. 

 Following amnion formation, all other parts of the outer 

 layer of the blastoderm, as in birds, form the serous membrane, 

 in which the amnion and yolk sac are included" (II 9r, 

 255-256 (192)) (Figure 29, 24). Baer noticed that attention 

 was not given to the serous membrane, and thus it was impossible 

 to understand the development of membranes. 



The urinary (allantoic) sac of mammals, Baer observed, 

 grows when body of the embryo is opened nearly the whole 

 length. It represents the growth of cloaca and is composed 

 of two layers: an internal layer which is the continuation 

 of the serous layer, and an external or vascular layer. Into 

 the latter enter two branches and ends of veins, passing by 

 the lower edge of abdominal layers. These vessels are the 

 future umbilical arteries and veins, which share in the 

 formation of chorion and placenta. The subsequent development 

 of the allantoic sac of carnivora (similar to birds) leads 

 to its growth, so that it extends through the back of the 

 embryo from right to left and meets with its opposite part, 

 leaving free only the place for the vitelline vesicle. In 

 the internal half of the allantoic sac, adjacent to the 

 amnion, the blood vessels are few, and in the internal half 

 which intergrows with the superficial membrane, forming the 

 chorion, the vessels grow and form the vascular network of 

 the placenta. The case is different in hooved animals. The 

 allantoic sac widens a little and lies near the amnion. 

 Because it is more elongated, it digs through the external 

 layer and goes beyond its limits. The layers of the allantoic 



410 



