202 Mr. A. Hyatt on the 



increased by more frequent contact with food and by being 

 constantly occupied in the act of ingestion. The differentia- 

 tion of the cells having been thus established and kept up by 

 a continuance of similar habits, and the aula correlatively 

 developed, Ave should have a free moving form with the cells 

 at one pole feeding-cells, and at the other probably more effi- 

 cient 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 exist- 

 ence 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 does not weigh against the observed functions of 

 the collars and flagella of the Flagellata, especially the 

 positive and convincing proofs brought forward by Saville 

 Kent. 



The parenchymula is a recently discovered stage of the 

 embryo immediately succeeding the closed blastula. The 

 esoteric cells differentiated during preceding stages have 

 been found by several authors to quit the exterior, where 

 they originated, and wander into the interior, where they 

 presumably give rise to the endoblastic cells subsequently 

 found there. 



A differentiated colony, like the amphiblastula, with the 

 cells at one end becoming better fitted to take in food, could 

 be transformed into a parenchymula by the migration of 

 differentiated feeding-cells into the interior, and the paren- 

 chymula could thus have been transformed into a true gastrula. 

 There are no living forms, so far as we know, with which 

 the parenchymula can be compared, and its probable 

 meaning has already been indicated by other writers, espe- 

 cially by MetschnikofF, namely, that it implies a radical form 

 in which the mesenchyme has arisen as a primitive mass by 

 delamination. 



The inwandering of the esoteric cells of the parenchymula 

 might be reasonably assumed as in part due to pressure. 

 This appears to be a primitive mode of forming the endoderra 

 as stated by Schmidt and MetschnikofF, and therefore we 

 should have to consider pressure as simply a possible cause 

 aiding the tendency to inwandering, as it appears in the habits 

 of these cells of the parenchymula. It is possible that this 



