No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 441 



Professor Wilson also pointed out that in all three groups the 

 first cleavage of the first and third quartettes occurs in the same 

 direction, and that the cleavage of the gasteropods and the anne- 

 lid is characterized by an additional point of agreement, in that 

 the primary mesoblast cell, \d, is given off in both cases in a 

 left-handed spiral, from the left posterior macromere D. 



It was found later by Lillie ('95) that the cleavage of Unio, 

 one of the lamellibranchs, agrees with that of the gasteropods 

 and annelids, not only in the features mentioned, but in several 

 other points of undoubted morphological significance ; and Conk- 

 lin has since added many other striking resemblances which 

 characterize the cleavage of annelids and gasteropods. The 

 discovery made by Conklin that the turret cells in Crepidula 

 have the same origin, position, and, in the main, the same fate 

 as the trochoblasts of the annelids, and that the prototroch, in 

 both groups, is completed by certain cells of the second quar- 

 tette, affords one of the most notable points of similarity in 

 the cleavage of these forms. 



Resemblances such as these would seem to indicate, as Conk- 

 lin strenuously insists, that cleavage has a much greater mor- 

 phological significance than has usually been assigned to it. 

 Yet, while recognizing all these wonderful similarities between 

 the cleavage of different classes of animals, does it follow that 

 the form of cleavage has any really fundamental connection 

 with the process of development .-' While it is true that there 

 are numerous cases in which cells of corresponding origin and 

 position have the same fate in widely separated groups, there 

 are other instances in which cells of the same origin have a 

 very different fate in forms which are much more closely allied. 

 (Compare the fate of the larval mesoblast cell in Unio with that 

 of the same cell in Planorbis.) And there are also cases in 

 which it has been found that cells which have the same fate 

 have a quite different origin, even in the same class of animals ; 

 instance the origin of the secondary mesoblast in Crepidula and 

 Planorbis. It is hard to reconcile these facts on the view that 

 the process of cleavage has any fundamental connection with 

 the homology of organs. Neither can the similarity in the 

 cleavage of different groups be accounted for by attributing it 



