224 BIOLOGY: GENERAL AND MEDICAL 



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- 

 tem and eyes, provision for which is made very early, 

 at which time all vertebrate embryos look much 

 alike. All of the vertebrate embryos have long tails 

 (see the Romanes diagrams). The ventral surface 

 becomes closed by the concrescence of buds that form 

 the face and neck and the thoracic and abdominal walls. 

 The branchial region of all vertebrate embryos show 

 slits or folds where the gills of the fishes and batrachians 

 are formed, though the mammalian embryos do not 

 have real gill clefts at this time. In all cases the limbs 

 first appear as buds that grow into shapeless excrescences, 

 which subsequently elongate, differentiate, and become 

 perfected, it being impossible to tell the final character 

 of these members for some time after they have first 

 appeared. 



The tail persists for a surprisingly long time in the 



