CHAPTER IV 



THE RELATIONSHIP OF EXISTING ORGANISMS 



{Continued) 



2. EMBRYOLOGY OF VERTEBRATES 



The relationships of no animals are more striking ban those 

 of the vertebrates, and no phase of vertebrate relationship is more 

 fascinating than the similarity of their embryonic development. 

 This field has a twofold bearing on evolution, for it illustrates 

 actual relationships in the formative stage of the individual and 

 at the same time points strongly to the probable succession of 

 changes which has brought about the transition from lowest to 

 highest. 



Our knowledge of embryology has contributed so conspicuously 

 to the science of comparative anatomy that it is difficult in many 

 cases to separate the two; indeed, the latter cannot adequately 

 be treated without the inclusion of a considerable body of facts 

 from embryology. For this reason the matter presented here must 

 partake somewhat of both fields. It falls into two categories, (1) 

 the resemblance of embryos of the different classes and (2) the 

 resemblance of embryos to the adults of lower classes. 



The Resemblance of Embryos of Different Classes 



The individual first becomes an entity when the single cell from 

 which it is to develop is matured, whatever may be the nature of 

 this cell. From this point to the completion of its body it is an 

 embryo. Completion does not necessarily mean birth, for most 

 embryos are complete organisms long before birth. Nor does it 

 mean the realization of all the possibilities of differentiation in- 

 herent in the individual, for many of these are not normally ex- 

 pressed until long after birth. It means rather the formation of 

 those organs which constitute a complete individual, whether or 

 not they may later atrophy or undergo further development as life 

 proceeds. 



Cleavage. The first step in development has already been 

 mentioned as cleavage, or the splitting of the fertilized ovum into 



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