No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 23 



What is the fate of the second generation of ectomeres ? 

 The answer will be given separately for d"" and a}"" ; the others 

 form the larval mantle, or, at least, contribute to its forma- 

 tion. This being so, we have a satisfactory explanation of 

 their large size, which is due to the precocious segregation of 

 this large and important organ in single cells. The text 

 figures on p. 59 show the relation of these blastomeres to the 

 future embryonic areas. 



5. Establishment of Bilateral Symmetry. The Larval Mesoblast. 



The spindle of bilateral division of the first somatoblast is 

 seen in Fig. 44 (PI. IV). This is a view from behind of a 

 stage slightly older than Fig. 42 (PI. IV). Shortly after the 

 completion of the division indicated, each of the resulting cells 

 buds forth a small cell x^ towards the vegetative pole (PI. IV, 

 Figs. 45, 46, and 47). These two small cells are placed just 

 behind x^-^ and x^ (PI. IV, Fig. 45) and form with the cells x^, 

 x^-^, x'^, and x'- the begimiing of a tongue of cells, which grows 

 forward and over the second somatoblast. Between the ento- 

 meres and the two cells X, X {the protoblasts of the shell-glajid) 

 there are no cells after the inclusion of the mesoblasts but the 

 derivatives (x'-x^) of the first somatoblast. Therefore there is 

 no room for doubt, that, after the invagination of the entomeres 

 and inclusion of the mesoblasts, all of the cells lying between 

 the blastopore and the posterior end of the shell-gland are 

 derivatives of the first somatoblast. I anticipate here what 

 will be shown in detail later, viz. : that these cells form the 

 ciliated plate (Wimperschild of Flemming) from which the foot 

 is derived later on. 



The next bilateral division is that of the second somatoblast 

 (mesoderm proteloblast) M (PI. IV, Fig. 45). Fig. 46 shows 

 the two mesoblasts just after the completion of this division. 

 The invagination of the entoderm is slightly indicated in this 

 figure, and its position is sufficient proof that the cells con- 

 cerned are A, B, C, and D. 



With these two divisions {i.e., of the first and second somato- 

 blasts) the bilateral character of the cleavage becomes apparent. 

 By this I do not mean to say that there is a complete cessation 



