106 MORPHOLOGICAL DEVELOPMENT. 



able proposition ; and had we no evidence beyond that which 

 adult vertebrate animals of developed types supply, it would be 

 a proposition not easy to substantiate. But abundant support 

 for it is to be found in the structure of the vertebrate embryo, 

 and in the comparative morphology of the Vcrtebrata in 

 general. 



Embryologists teach us that the primordial relations of 

 parts are most clearly displayed in the early stages of evo- 

 lution; and that they generally become partially or com- 

 pletely disguised in its later stages. Hence, were the verte- 

 brate animal on the same level as the annulose animal in 

 degree of composition did it similarly consist of segments 

 which are homologous in the sense that they are the prox- 

 imate units of composition ; we ought to find this funda- 

 mental fact most strongly marked at the outset. As in 

 the annelid-embryo, the first conspicuous change is the 

 elongation and division into segments, by constrictions that 

 encircle the whole body; and as in the articulate embryo, 

 the blastoderm becomes marked out transversely into pieces 

 which extend themselves round the yelk before the internal 

 organization has made any appreciable progress ; so in the 

 embryo of every vertebrate animal, had it an analogous com- 

 position, the first decided change should be a" segmentation 

 implicating the entire mass. But it is not so. Sundry im- 

 portant differentiations occur before any divisions begin to 

 show themselves. There is the defining of that elongated, 

 elevated area with its longitudinal groove, which becomes the 

 seat of subsequent changes ; there is the formation of the 

 notochord lying beneath this groove ; there is the growth 

 upwards of the boundaries of the groove into the dorsal 

 laminae, which rapidly develop and fold over in the region of 

 the head. Rathke, as quoted and indorsed by Prof. Huxley, 

 describes the subsequent changes as follows: "The gelatin- 

 ous investing mass, which, at first, seems only to constitute 

 a band to the right and to the left of the notochord, forms 

 around it, in the further course of development, a sheath, 



