by the remnant of the serous membrane h t separated from the 

 yolk sac. The fate of the serous membrane becomes clear 

 upon comparison with the earlier stage when the amnion is 

 in contact with the serous membrane, and when the allantoic 

 sac remains very small. The allantoic sac of a dog's 

 embryo, developing from the cloaca at the age of about three 

 weeks, covers the embryonic body touching the external 

 surface of the amnion and internal surface of the chorion. 

 The vessels of the external layer of the allantoic sac form 

 the villi. Later, the embryonic sac stretches into the 

 form of cylinder with rounded ends, and the villi forms the 

 belt-like placenta. The yolk sac of carnivores is rich in 

 vessels, and its duct remains open for a long time. 



The embryonic sac of swine (Figure 29, 22) is repre- 

 sented at the moment when the formation of chorion continues 

 (up to the end of the second week) ; the formed embryo is 

 seen with the allantoic sac coming out from it. The latter 

 appears earlier in thick-skinned animals than in carnivores 

 and grows very quickly. Umbilical arteries enter the 

 allantoic sac from the two sides of the embryo, and the 

 umbilical veins go out into ramifications in the abdominal 

 wall. After the formation of anastomosis between the umbil- 

 ical veins, the right vein gradually disappears. In the 

 external membrane of the embryonic sac the blood vessels 

 appear only when the chorion is formed, or after the 

 accretion of the membrane with allantoic sac. In swine 

 embryos, Baer followed in detail the development of villi 

 of the chorion. At first the beginning skin transverse 

 fold lines appear with a height of 0.1; the free edges of 

 folds are covered by cuts that lead to the formation of 

 many villi. The same process takes place in the internal 

 surface of the placenta, while between its villi arise 

 the villi of the chorion. Also, Baer followed up the 

 process of the union of the allantoic sac with the external 

 membrane of the embryonic vesicle and the intergrowth of 

 blood vessels in the embryonic vesicle with the formation 

 of vascular networks in the villi. After the end of the 

 fourth week, the network of the chorion's blood vessels 

 increases, filling all the villi and spaces between them; a 

 denser network of vessels develops in the internal wall of 

 the uterus. 



416 



