No. 3] PODARKE OBSCURA VERRILL. 459 



homologous adult parts may arise from non-homologous Anla- 

 gen ; and, conversely, cleavage cells having the same origin 

 give rise to non-homologous organs. While most of his lecture 

 deals with embryonic structures which appear after the cleav- 

 age, it is, I think, evident that Professor Wilson intended to 

 discredit the study of cytogeny as a means of determining 

 homologies. 



Mead (No. 22) devoted considerable space to a discussion of 

 this question of cell homology, which he defined as follows : 

 " Essential similarity in origin and fate will be considered a 

 sufficient criterion of the homology of cells, as it is, by common 

 consent, of tissues and organs." Mead first shows that in 

 general the mode of origin and the fate of the quartettes of 

 ectomeres are the same in all annelids and mollusks. Then, 

 examining the cleavage more in detail, he finds that in Amphi- 

 trite and Clymenella the primary and secondary trochoblasts 

 are cell for cell the same ; and, further, in Scolecolepis, which 

 has a suppressed trochophore, these very cells are diminutive, as 

 might be expected from a non-functional organ. This proto- 

 troch formation is very different from the process as described 

 for Nereis, but Mead gives reasons for believing that Wilson's 

 description was erroneous. 



Mead further shows that the objection- raised by Lillie to 

 cell homology because of the varying fate of cell 3a2.2 is not 

 valid, because in neither case was the whole of this cell in- 

 volved. In both cases it divides horizontally, and while the 

 lower portion in Nereis becomes stomatoblast, in Unio it is the 

 upper portion which becomes larval mesoblast. 



Another case of cell homology is the apical rosette, which 

 probably has exactly the same origin and fate in all the anne- 

 lids studied, and in Crepidula among the mollusks. The slime 

 glands of Amphitrite also are homologous in position, and prob- 

 ably in fate, with the " head kidneys " of Nereis. 



The fate of the remaining cells of the upper hemisphere was, 

 according to Mead, so uncertain that he did not attempt to 

 follow homologies, and the same was true of the lower hemi- 

 sphere, except, in a general way, the somatoblast 2d, and, more 

 especially, one of its descendants, the small cell, X1.2, which 



