No. I.] THE EMBRYOLOGY OF CREPIDULA. 103 



The annelidan cross (rosette series) has also been described 

 by Mead ('94) in Amphitrite, Lepodonotus, and Clymenella; 

 and in the case of the first two, the division of the "four cells 

 which form the cross is bilateral," as it is in Crepidula, whereas 

 in Nereis it is radial. 



We find, therefore, in a large number of annelids and mol- 

 lusks, representing very widely different orders, that there is a 

 peculiar arrangement of cells at the apical pole which takes the 

 form of two crosses whose cells have in the different animals 

 the same origin, the same axial relations, and (as will be shown 

 later), in many cases at least, the same destiny. 



What is the significance of these crosses } Are they the 

 necessary result of alternating cleavages, surface tension, and 

 the like, or must we seek their cause in more remote and 

 obscure phenomena.'' As was just mentioned, Wilson finds 

 that their " origin is in some way connected with the mechani- 

 cal conditions of cleavage." This conclusion is unquestionable, 

 but that their characteristic and peculiar features are entirely 

 caused by such conditions, as such a statement might seem to 

 imply, is very questionable indeed. 



In the first place it may be well to call attention to the fact 

 that these radiating structures are not caused by the crowding 

 of cells into the furrows between the macromeres. This is well 

 shown in Neritina, Paludina, Crepidula, and Nereis, in which 

 the cross is first formed midway between those furrows. In 

 Umbrella, indeed, it is first seen lying over the furrows 

 between the macromeres, — a position which it ultimately 

 takes in Crepidula, owing to the rotation of the entire cap of 

 ectoblast, — but there is no probability that even in Umbrella 

 the cross is caused by those furrows. 



The cross and the rosette series are the direct result of the 

 position, size, and shape of their constituent cells. Anything 

 that will satisfactoril)- explain these three things will afford a 

 satisfactory answer as to the significance of these structures. 



The position of cells in general is due to the direction of the 

 cleavages, and to the subsequent rotations of the division pro- 

 ducts. The shape of cells also may be explained in general as 

 the result of their size and position. The size of cells, however, 



