THE GASTRiEA THEORY AND ITS SUCCESSORS. 99 



the two extremities of the mouth of the Polyps, but a 

 discussion of this theory would occupy an undue amount 

 of time. 



I have hitherto omitted all reference in this discussion 

 to the Arthropoda and the Vertebrata. In the former 

 group a considerable amount of study of the early 

 stages of development is still necessary before the 

 processes of formation of the germ layers can be 

 homologized with accuracy with those of other Metazoa. 

 In one group, however, that of the Arachnida, the re- 

 searches of Metschnikoff on Chelifer, and Morgan on 

 the Pycnogonids, demonstrate the formation of an inner 

 layer of cells by delamination, and it seems not improb- 

 able that typical centrolecithal segmentation may be 

 derived from such a process by the extensive accumula- 

 tion of food-yolk in the inner portions of the blastula 

 cells. 



As reo^ards the Vertebrates there is an almost uni- 

 versal concensus of opinion among embryologists that 

 the peculiarities of their endoderm formation are to be 

 explained on the hypothesis of an ancestral invaginate 

 gastrula. It seems quite possible that invagination 

 secondarily derived from epibole may have become so 

 impressed upon the ontogeny of the Vertebrate ances- 

 tors as to leave its imprint on the development of the 

 later groups. 



There is still another point to be considered in con- 

 nection with the Parenchymella theory, viz., the causes 

 which operated to bring about the transformation of the 

 solid planula into the hollow gastrula. Metschnikoff's 

 views on this are as foUows : — It is clear that it would 

 be to the advantage of locomotor colonies of more or 



