but with the embryo still supplying nutrition and constituting 

 one unit with those extra- embryonic parts; 4) the inclusion of 

 the individual extra- embryonic parts of the rudiment within 

 the embryo (by immersing the remaining yolk in the embryo's 

 abdominal cavity) and the separation of the embryo from the 

 remaining parts (amnion and allantoides) upon hatching. By 

 this, according to Baer, "the last stage of the growing 

 independence" is attained. 



His comparison of the development of birds amphibia, fish 

 and mammals led Baer to conclude that in amphibia from the 

 beginning, the embryo is very large in comparison to the yolk 

 mass; to achieve independence, it does not require separation 

 (unlacing) as other embryos do. Bony fish hold an intermediate 

 position between the birds and amphibia in this respect, with 

 only insignificant unlacing. In mammals, the separation and 

 the covering with membranes takes place more quickly than in 

 birds and results in an especially long umbilical cord in the 

 most highly organized representative of mammals, the human. 



This early separation of the mammalian embryo from the 

 yolk sac does not contradict the truth that for all vertebrates 

 the embryo constitutes a single unit with the blastoderm and 

 yolk. Fertilization of the egg, which was previously a part 

 of the mother, produces an independent unit, similar to the 

 parents; the features of this unit appears during the process 

 of development. 



In lower animals, according to Baer, there is no separation 

 of sexes, and reproduction occurs only by growth beyond the 

 limits of the individual. Baer developed this erroneous idea 

 in the special supplement to his second scholium in "Corollaries 

 about reproduction." There he tried to discredit the data about 

 parthenogenesis in vertebrates, and with perplexity he dealt 

 with facts about the parthenogenetic development in the plant 

 louse and certain butterflies. The difficulties which Baer 

 met in the interpretation of features of fertilization he 

 explained with the standard knowledge of the time. 



The third scholium, "Interior transformation of individuals," 

 concerns those routes along which the embryo develops. Baer 

 stressed that individual development at all stages proceeds 

 from the homogeneous to the heterogenous, from the general to 



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