462 TREAD WELL. [Vol. XVII. 



Conklin further distinguishes between cell and regional homol- 

 ogy, the latter appearing, for example, between annelids and 

 gasteropods on the one hand, and the ctenophores on the other. 

 Further, if I understand him correctly, although he gives no 

 formal definition, he would distinguish between complete and 

 incomplete cell homology. I am able to see very little dis- 

 tinction to be logically made between an incomplete and a 

 regional homology, and it seems to me a clearer definition of 

 cell homology is to be desired. If cell homology is the resem- 

 blance which exists between two organs whose Anlagen are 

 merely similar and not idejitical in origin, I have no objection 

 to the use of the term in recent writings ; but it seems to me 

 very little distinction can be made between this and regional 

 homology, and I see no reason why, using this definition, all 

 homologies are not cell homologies. The phrase " incomplete 

 cell homology " is to me a contradiction of terms. 



I have no desire to split hairs over a mere definition, and it 

 may be that the term is convenient and should be retained (for 

 example, to designate an homology between two organs, not 

 determined by adult structure alone, but by a similarity in 

 cellular origin as well). At the same time it seems to me 

 desirable to emphasize the distinction between the complete 

 and the incomplete, as the hasty reader of many recent cell- 

 lineage papers would, I think, be in danger of carrying away 

 too exaggerated a notion of the number and importance of the 

 former. To this point I shall return. 



Wilson, in a paper already quoted, has reinvestigated the 

 platode cleavage, and has shown that although the cleavage 

 in its general character agrees perfectly with the annelid 

 and gasteropod, yet the mesoblast, precisely as described by 

 Lang for Discocoelis, arises from the second and possibly, 

 to a limited extent, from the third, quartette of ectomeres. 

 While this breaks down Mead's criticism of Lang, it at first 

 sight seems to revive the old difficulty in the way of accept- 

 ing homologies between the two groups. In view of the recent 

 discovery of larval mesoblast in annelids, gasteropods, and 

 lamellibranchs, however, Wilson believes that the apparent 

 contradiction is in reality new proof of the homology as showing 



