The formation of the umbilicus of the mammalian embryo 

 takes place as in birds. Concerning the umbilical cord, in 

 the majority of mammals the yolk duct dies off early and 

 the vessels of the yolk sac disappear. The length of the 

 umbilical cord is not equal in different mammals; man's is 

 the longest, it is slightly shorter in monkeys, followed by 

 that of hooved animals, and with the rodent's shortest. 



Description of mammalian development he concluded by 

 counting the embryonic parts lost during birth, the chorion, 

 amnion, yolk sac and placenta with the umbilical cord. 

 These parts in mammals are correspondingly larger than those 

 in birds. 



In a special section, Baer described the structure and 

 development of the embryonic sac in different mammals, 

 especially in humans. The preliminary remarks which open 

 this paragraph stress the importance of comparative embryo- 

 logical investigation of the embryonic sac and the history 

 of development of embryonic membranes. Thus "exact knowledge 

 of the different mammalian forms of ovarian membranes can 

 help the understanding development of human embryonic 

 membranes. The first steps of their formation are unavail- 

 able for investigation" (II 10 a, p. 313 (235)). For 

 explanation of the text, Baer offered diagrammatic illustra- 

 tions. The first of these (Figure 29, 19) represents a 

 transverse section of a bird's egg about the eighth day of 

 incubation, with which the embryonic ova of different mammals 

 are compared. In this drawing, a is a section of the embryo, 

 b the amnion. In the body of the embryo are seen the primary 

 kidneys, mesentery and intestine, with the yolk duct leaving 

 at c, which passes to the yolk sac d. On the latter the 

 distribution of vessels and the boundary vein are shown. The 

 allantoic sac still occupies only part of the ovum; its 

 external half /, adjacent to the shell membrane, marks the 

 beginning of the chorion, and the internal half g surrounds 

 the amnion. The allantoic sac already extends over the back 

 of the embryo on the opposite side. Further, the allantoic 

 duct e, remnants of the serous membrane h and the albumen of 

 the egg i are also represented. 



The embryonic sac of carnivores (Figure 29, 21) is shown 

 in the stage of the already-formed chorion and developing 

 placenta. The yolk sac, with an elongated form, is surrounded 



415 



