20 LILLIE. [Vol. X. 



vegetative pole. Before passing on to a description of these 

 changes, I will direct attention to another division of X (PI. Ill, 

 Figs. 31 and 33), by which a small cell is budded off towards 

 the apical pole just posterior to ^'-^ (PI. Ill, x^, Fig. 35). The 

 appearance of this cell at this time and place is extremely 

 interesting, seeing that it tallies exactly with Nereis (E. B. 

 Wilson I.e., PI. XVI, Fig. 33). 



The remaining members, a^, b^, and C^, of the third group of 

 ectomeres are now formed by the simultaneous leiotropic 

 division of A, B, and C (PI. Ill, Figs. 32 and 34). After these 

 divisions the entoderm is definitely localized in the four cells 

 A, B, C, and D (PI. Ill, Fig. 38). The larger part of D is, 

 however, mesoderm, which in the next figure (39) is shown 

 about to be definitely separated. Whe^i this division is com- 

 pleted {PL IV, Figs. 41 and 42) the delimitation of the germinal 

 layers in disti^ict blastomeres is accomplished. The other 

 spindles in Fig. 39 explain themselves, and the resulting cells 

 are shown in Fig. 42. 



The embryo at the time of the separation of the germ-layers 

 contains thirty-two cells (see table of cleavages, p. 33). At 

 the same time the embryo of Nereis contains thirty-eight cells. 

 The difference is due to the suppression of cleavage in the 

 apical pole cells of Unio. The composition of the embryo can 

 be gathered from the following table : 



Entomeres. A — D 4 



Mesoblast. M {d^) i 



Larval Mesoblast. Y and j)/' 2 



Ectomeres of first generation 10 



•Ectomeres of second generation {b^-^, b^-^, c^-^, ^2-2 and a^-^) 5 



First Somatoblast 6 



Ectomeres of third generation 4 



32 



The number (32) of cells at this stage is a chance coinci- 

 dence merely ; it has not been reached by a geometrical pro- 

 gression from two, four, eight, sixteen, to thirty-two cells, as 

 in a synchronously cleaving ovum. 



Almost every cell in this stage has so distinctive a character 

 that if isolated from the cell-complex it could be recognized 



