No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 



The second plane of division is likewise meridional, and 

 practically at right angles to the first. The two cells divide 

 at diiferent times, and these two divisions taken together rep- 

 resent what is usually called the second furrow in other eggs. 

 In the two-cell stage then there arises a certain independence 

 in the times of cleavage of the blastomeres. Later when one 

 cell gains a start, so to speak, over the corresponding cell of 

 another quadrant, it continues to maintain or may even increase 

 its lead. This difference in the time relations of the divisions 

 of the cells of different „ . 



quadrants is one of the ^ Vti%\. ' 



most striking features 

 in the cleavage of 

 Unio. The four cells 

 of a quadrant never 

 divide synchronously ; 

 the difference in time 

 may be slight but it 

 remains constant 

 throughout many divi- 

 sions. 



CD is the first to di- 

 vide ; the products of 

 division are unequal 

 (PI. I, C and A Figs. 

 6 to 9). The smaller 

 cell C lies on the right 

 side of the future em- 

 bryo ; D is much larger and occupies the posterior end of the 

 embryo. During the division of CD, AB begins to constrict 

 (PI. I, Fig. 6), and shortly after the separation of C and D 

 divides into two approximately equal parts (PI. I, Figs. 8 and 

 9). Of the four cells now making up the embryo A, B, and C 

 are approximately equal ; j5 is as a rule slightly smaller than 

 A ox C \ D is very much larger than any of the other three. 

 (Text — Fig. I.) 



This early division of CD is the first indication of a tendency 

 to progressively more rapid cell multiplication in the posterior 



Ant 



m'lp 



Fig. I. — Four-cell stage, mlp ^ median longitudinal plane 



, , I — I = First cleavage-plane, 



of larva „ . , , 



2 — 2 = Second cleavage-plane. 



