No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 19 



the division is r', a^, and b^, thus repeating the order of the 

 first divisions of D, C, A, and B} Figs. 22 and 23 (PI. II) 

 illustrate the appearance of the apical and vegetative poles 

 respectively in the seventeen-cell stage. The seventeenth cell 

 is x^ (Fig. 23), which has been budded forth from X just 

 behind C on the vegetative pole. (Cf. Figs. 20 and 24 of 

 Nereis ; Wilson I.e.) The first division of the first generation 

 of ectomeres is leiotropic. 



4. From the Seventeen to the Thirty-eight-cell Stage: Third 

 Generation of Ectomeres, etc. 



The eighteen-cell stage is reached by another division of D 

 (PI. II, Fig. 23). The cell to the left is the first member of 

 the third generation of ectomeres {d^, Figs. 25 and 27). A 

 period of rest now ensues. When activity is again resumed, 

 spindles appear almost simultaneously in «^ /5% c"", and X {d^). 

 The positions of these spindles and, in consequence, of the 

 resulting cells is very different in the different cases. Figs. 

 24, 25, and 27 (PI. II) illustrate the description. A cell, ;r^ is 

 budded off from X on the left side, symmetrical with ;f' on the 

 right side and just posterior to d^ (PL III, x^, Fig. 29). The 

 cleavage of c^ and b^ is equal and dexiotropic. The division of 

 a" calls for special attention, inasmuch as the larval mesoblast 

 is separated by it. Fig. 25 shows that the division will be 

 equatorial. After the completion of the division it is seen that 

 the cell nearer the vegetative pole (PI. Ill, «^-^ or Y, Fig. 29), 

 which becomes the larval mesoblast, is the larger. Y is 

 bounded by the following cells : on the right by x^ and d^, on 

 the left by d^-^ and a^\ behind by X, and in front by A and 

 part of D. Already (PI. Ill, Fig. 29) it is partially covered by 

 x^ and d^. 



During these divisions the cells of the apical pole lose their 

 rounded contour and together form a flat plate of cells (PI. II, 

 Fig. 26). They now enter upon a long period of rest, during 

 which extremely important changes, which lead to the estab- 

 lishment of the mesoblast and entomeres, take place at the 



^ Lang has observed that in Discocoelis the first generation of micromeres follow 

 the same rhythm of division as their parent macromeres (No. 53, p. 331). 



