694 



PROCEEDINGS OF THE ACADEMY OF 



[Oct., 



ing much in advance of A-B, as fig. 5 shows. As seen in the figm-e, the 

 spindle in C-D has reached the late anaphase, the chromosomes lying 

 close to the centrosomes, while a double row of microsomes has ap- 

 peared on the spindle fibres. On the other hand, in A-B the spindle 

 has only just reached the metaphasc. The division of A-B is nearly 

 equal, A being slightly the larger product, while the division of C-D is 

 highly unequal. The spindles for both divisions arc inclined in a 

 leiotropic direction, as is especially well shown by the division of 

 A-B in fig. 6, in which the left pole of the spindle is much higher than 

 the right. 



The four cells formed by the second cleavage all differ in size (figs. 6 

 and 7, text fig. II, A and B); while D is relatively colossal, C, B, and 

 A are more nearly alike. Of these C is the largest, A slightly smaller 

 than C, while B is the smallest of all. The enormous size of D can be 



A B 



Fig. II. A, 4-cell stage from animal pole; B. same from right side. 



appreciated only when the 4-cell stage is viewed from the side, as shown 

 in text fig. II, B. 



The size relation of the four cells A, B, C and D appears to be some- 

 what unusual. In all Annelida and Mollusca investigated which have 

 imequal cleavage the left posterior blastomere D is the largest one of 

 the 4-cell stage, except in the case of Aplysia (Blochmann, 1883; Ca- 

 razzi, 1900) and the Pteropoda thecosoniata (Fol, 1875). A and B are 

 usually equal, while C is more or less intermediate between B. and D. 

 Among the Lamcllibranchia there appear to exist cases parallel to that 

 of Dinophilus. An examination of the figures given for Cyclas (Stauff- 

 acher, 1893), for Unio (Lillic, 1895) and for Dreissensia (Meisenheimer, 

 1901) seems to indicate that in these forms B is the smallest of the four 

 cells, while it is fairly evident that C exceeds A in size, although I find 

 no precise statement of these facts in the text of the papers mentioned 

 above. In all these forms D is relatively enormous. 



