Hyatt.] 154 [March 5, 



bryos of sponges is entirely inadequate to prove anything but 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. 



This view is quite similar in its result to the opinion of Balfour 

 (Comp. Embr. vol. i, p. 122) which was also founded upon the 

 idea that the Porifera presented characteristics of a more primitive 

 kind than was usual in the higher types of Metazoa. Balfour rec- 

 ognized a difficulty in the invagination of the ciliated cells in Sy- 

 cones, and thought that the possession of cilia was an essential 

 character implying respiratory and locomotive functions. Balfour 

 perhaps laid too great stress upon the similarity of functions, and 

 this led him to suppose that the three layers in sponges might pos- 

 sibly prove to be distinct from the three layers of other Metazoa 

 {ibid. p. 123), whereas they are truly homologous. 



A dimorphic 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 parenchymula could as we have tried to 

 show become 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, 

 especially by Metschnikoff, namely, that it implies a radical form 

 in which the mesenchyme has arisen as a primitive mass by delam- 

 ination. 



"We have also claimed that the gastrula of the Porifera was of 

 general and genetic significance. This possibly indicates the 

 former existence of an amphiblastula-like ancestor in which the 

 invagination of the esoteric layer arose from the pressure occas- 

 sioned by the unequal growth of the hemispheres, as first suggested 

 by Dr. Whitman. This theory is satisfactory, so far as pressure 

 may be considered as assisting in the first introduction of the ten- 

 dency to invagination, but as a purely mechanical theory it cannot 

 fully explain the whole series of phenomena. The aid of concentra- 

 tion in heredity is essential in order to account for the early ap- 

 pearance of both embolic and epibolic gastrulae in the embryos, and 

 for the skipping of these stages in cases of the formation of the 

 endoderm directly by delamination. The inwandering of the es- 



