human embryos showed that the embryo begins to form in the 

 internal sac, as all other mammals do. Then the embryo 

 untwists, so that the remaining part of the sac produces 

 the beginning of the yolk sac, which is called in human 

 beings the umbilical sac. It quickly moves away from the 

 embryo, while connected with it by the yolk duct. The 

 umbilical sac usually disappears in the third month of 

 uterine life. It may be inferred by analogy with birds 

 and mammals that the amnion of the human embryo grows 

 extremely quickly, even though the formation of the human 

 amnion was not observed. Baer asserted that in human beings 

 its development "must not take place differently" (II 10 q, 

 p. 363 (233)). Direct observations, showing the ingrowth 

 of blood vessels from the wall of the allantoic sac to the 

 external membrane of embryonic vesicle, forced Baer to 

 consider the comparative embryological suggestion that the 

 human chorion is formed as in other mammals. Baer considered 

 the question about the structure of the human allantoic sac 

 controversial, but he did not doubt that, here also, the 

 principal stages of its development are the same as those 

 of other vertebrates. The villi of the human chorion are 

 longer, thinner and more ramified than those of other 

 mammals. They are entwined by a network of vessels to form 

 placenta. The development of the umbilicus and the human 

 embryo itself was not described by Baer, who referred this 

 aspect of humans to other mammals (105) . 



The concluding section of his third part, Baer entitled 

 "Development of animals which have no amnion and no yolk sac" 

 (II § 11, p. 280). Mentioning the similarity characterizing 

 development of reptiles, birds and mammals, Baer indicated 

 that fish and amphibia possess essential differences in the 

 structure of the ovum and membranes. This forced him to 

 recognize that the lineal group of amphibia must be divided 

 into two classes, amphibia and reptiles. 



The main peculiarity of fish and amphibia is that they 

 are always deprived of the amnion and allantoic sac. Instead 

 of the latter, fish and reptiles develop other organs of 

 respiration, the external gills. 



418 



