236 BIOLOGY: GENERAL AND MEDICAL 



Minot says, " The embryo is not a correct or adequate 

 record of the ancestral type, 



1. Because the embryos have necessities of 

 their own which have led to their modifica- 

 tion in the course of evolution; 



2. Because the embryos consist of undifferen- 

 tiated cells; 



3. Because the embryo at each stage must be 

 regarded as the mechanical cause of the 

 next and following stages. 



However, Minot declares these to be objections to 

 the theory rather than to the facts which he believes 

 fully justify the interpretation that ontogeny does re- 

 capitulate the phylogeny. 



Though the resemblances between embryos become 

 more and more pronounced as we follow them back 

 toward the egg, there is no time when embryos belonging 

 to widely divergent organsims are precisely alike. 

 Every embryo, at every stage of its development, is an 

 individual of the particular genus and species to which 

 it belongs and has at every stage peculiarities which 

 distinguish it from every other genus or species. It is, 

 however, invariably true that the more closely the 

 species are related, the greater is the resemblance of their 

 embryos at all stages, and the more widely they are 

 separated the further back it is necessary to go to find 

 the phylogenetic resemblances. 



If we examine the developing mammalian embryo 

 for phylogenetic resemblances, they are easily found. 

 Every embryo begins by segmentation resulting in some 

 kind of a morula; the morula always passes into some 

 kind of a blastula stage, and the blastula always under- 

 goes some kind of gastrulation. The beginning embryo 

 is always elongate and slender. The gut is always 

 formed by concrescence of the folded entoderm and 

 inclosed by concrescence of the externally folded ecto- 

 derm. The vertebrate embryo diverges from all others 

 by the preponderating importance of its nervous sys- 



