No. I.] 



THE EMBRYOLOGY OF CREPIDULA. 



127 



cells (3a2, 3b2, 30^, 3d2) divide at nearly the same time, and 

 soon after, the outer upper cells in the quadrants C and D (3c'-^ 

 and 3d'-^) divide in nearly a radial direction, while 3c'-' and 3d'-' 

 do not divide. In shape and position the cells resulting from 

 these divisions are almost identically the same in Umbrella and 

 Crepidula, though the method by which they arise is some- 

 what different in the two cases. 



Heymons has observed two further divisions of his cells c'-'-' 

 and d'-'-' ( E and Ei), i.e., of the two upper cells in the quad- 

 rants C and D; but since I have not observed these divisions in 

 Crepidula, I need not give a detailed description of them 

 here. 



The cleavage of the third quartette in Umbrella may be 

 compared, at a glance, with the cleavage of the same cells in 

 Neritina and Crepidula by means of the following table, which 

 gives the lineage of 3a and 3d in Umbrella: 



When we come to sum up the resemblances between 

 Umbrella and Crepidula in the history of the third quartette, 

 we find the same remarkable similarity which characterizes the 

 cells of the other quartettes. Owing to the fact that bilateral 

 symmetry appears in the posterior cells of this quartette (3 c 

 and 3d) the division of these two cells is highly peculiar, but 

 all these peculiarities (at least as to the cell products, if not as 

 to the method of their formation) are point for point exactly 



