340 A. Hyatt — Larval Theory of the Origin of Tissue. 



abling the cells to use both sides instead of one, and to perform 

 the functions of respiration, ingestion and excretion more com- 

 pletely is obvious. The growing of the cells of the ovum into 

 a hollow sphere, the blastula with its blastulapore opening ex- 

 ternally, is described by Butschli as essentially similar to the 

 growth of the adult floating spherical colonies of Volvox and 

 Eudorina from a single zoon by fission. This author (Bronn's 

 Thierreichs, Protozoa, pi. 45) gives a series of figures illustrat- 

 ing the development of the asexual zoons of Volvox which 

 fully substantiate his comparisons, and, together with Carter's, 

 show that the closest comparisons may be made between the 

 early stages of the ovum and those of all forms of Volvox, 

 which is an open blastula like that of some Porifera before it 

 leaves the parent colony and becomes free. 



All of these comparisons seem to be much opposed to Butschli's 

 supposition that the primitive cavity of the blastula originated 

 from a separation of two layers rather than as a stage of devel- 

 opment from one primitive layer and the formation of an aula. 



In order to account for the differentiation of the esoteric cells 

 we have imagined them, as necessarily by position feeding 

 cells in the ancestors of the diploplaculate stage. In the free 

 morula and closed blastula the same cells or their more modified 

 descendants would tend to retain similar functions. The differ- 

 entiation of the poles would occur in this blastula form accord- 

 ing to the same law, as is observed in the higher animals, and 

 the tendency already initiated of the zoons of one pole to be- 

 come exclusively feeding zoons would be increased by more 

 frequent contact with food, and by being constantly occupied 

 in the act of ingestion. The differentiation of the cells having 

 been thus established and kept up 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 remained quite capable of this office, as well, also, 

 as that of developing 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, with regard to these cells in the embryos of sponges is 

 entirely inadequate to prove anything except the fact that they 

 have not seen them actually feeding, and do not weigh against 

 the observed functions of the collars and flagelli of the Flagel- 

 lata, especially the positive and convincing proofs brought 

 forward by Saville Kent. 



