1884.] 153 [Hyatt. 



regarded as true endodermic cells produced suddenly by concen- 

 tration of development, the ovum having skipped the earliest 

 stages and also the gastrula. In other words, the mere fact that 

 these cells are permanently internal makes them for the first time 

 truly endodermic, and such cells are truly endodermic because they 

 have become internal through inheritance from some ancestral 

 gastrula. That is, we claim that the old and established laws 

 of homology apply not only to the organs and parts, but to the 

 primitive layers of ova and to the cell-zoons. Thus cells and 

 the primitive layers must occupy similar positions with relation to 

 each other in the body of the embryo before they can be considered 

 as strictly homologous, and, therefore, no external esoteric cells 

 can be homologized with internal endodermic cells, or be consid- 

 ered as their descendants. 



The closure of the blastulapore, which occurs in most forms, in- 

 dicates very clearly, that the internal cavity or blastocoel was not 

 a digestive sac in any sense. In order to account for the differen- 

 tiation of the esoteric cells, we have imagined them as necessarily 

 and by position feeding cells in the ancestors of the diploplacu- 

 late stage. In the free morula and closed blastula the same cells, 

 or their more modified descendants, would tend to retain similar 

 functions. The differentiation of the poles would occur in this 

 blastula form according to the same law, as is observed in the 

 higher animals, and the tendency already initiated of the zoons of 

 one pole to become exclusively feeding zoons would be increased 

 by more frequent contact with food, and by being constantly occu- 

 pied in the act of ingestion. The dimorphism of the colony hav- 

 ing been thus kept up and established by a continuance of similar 

 habits, and the aula correlatively developed, we should have a free 

 moving form with the cells at one pole feeding cells, and at the 

 other probably more efficient as respiratory cells. These last need 

 not be necessarily inefficient as feeding zoons, but might have re- 

 mained quite capable of this office, as well, also, as that of devel- 

 oping flagella for moving the body, and in fact resembling in 

 aspect and structure what we actually find in the amphiblastula of 

 some sponges. We here claim for the exoteric or ectoblast cells, 

 that their possession of collars and flagella implies the existence 

 of powers of ingestion. We think the negative evidence, adduced 

 by Metschnikoff and others, in regard to these cells in the em- 



