The embryonic sac of ruminants is in the essential 

 features similar to the ovum of the thick-skinned animals, 

 with the difference that the placentae of the ruminants are 

 multiple and differ in form in different species. The 

 embryonic sac of rodents (Figure 29, 20) is characterized 

 by a single placenta, confined to part of the ovum surface. 



In sloths (Baer mentioned the data of Carus and 

 Rudolphi) the placenta is elongated and rounded, consisting 

 of multiple placentae adjacent to each other. This forms a 

 remarkable transition between extremely unlike forms, namely 

 between ruminants and monkeys. 



The embryonic sac of the monkey is very similar to the 

 human sac, but more elongated in accordance to the form of 

 the uterus. The monkey umbilical sac is larger than that 

 of man, is also kept until birth, and possesses a similar 

 long stalk. The length of the umbilicus, which in monkeys 

 is larger than in all other mammals, also brings the monkey 

 nearer to man. 



Considering that he had to rely on the observations 

 of others for questions of human development more than for 

 other parts of his work, Baer at first refused to enter 

 into arguments concerning the nature of human beings. 

 "We shall," he maintained, "regard man only as a member of 

 the great animal kingdom" (II 10k, pp. 351-352 (264)). 

 Thus Baer determined his role as a naturalist with respect 

 to opinions on human nature. "Comparison with the develop- 

 ment of animals, and namely with the mammals," Baer continued, 

 "is considered the most reliable guiding star .... without 

 the flame of the comparative history of development we cannot 

 clarify the significance of the separate parts of early 

 embryonic sac of man" (II 10. k, p. 352 (264)). And further: 

 "You know the history of development of the mammalian egg and 

 can imagine that the history of the human egg represents only 

 a special case of the general history of the mammalian egg" 

 (p. 353 (II 10Z 265)) . 



The cover of the uterus is developed in human beings 

 earlier than in other animals; Baer claimed that he saw the 

 dropping off of the membrane on the eighth day of pregnancy. 

 This cover is supplied by fossae, in which the villi of 

 embryonic vesicle grow. The investigation of two-week-old 



417 



