No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 



25 



future difference of the two sides of the shell-gland. The four 

 resulting cells are seen faintly outlined in Fig. 61, which also 

 shows that the two anterior cells are somewhat the smaller. 

 By repeated divisions a plate of high columnar cells is formed 

 (PI. V, Figs. 64, 66, and 72), which occupies the whole of the 

 future dorsal region of the larva. It has caused an enormous 

 expansion of the area previously occupied by the cell X {cf. 

 PI. V, Figs. 59 and 72), and has thus completely altered the 

 embryonic topography {vide text figures, p. 59), establishing 

 the permanent dorsal axis of the larva and adult. 



Let us tabulate the divisions of the first somatoblast : 



Ventral plate. 



Right Half r>X^^ ' 

 OF THE - /^-^ 



Shell-Gland ^ 



Left Half 



OF THE 



Shell-Gland 



The cells referred to the center of the bar joining the two 

 central X's, are those which were budded from the first somato- 

 blast before its bilateral cleavage. The table indicates that the 

 first somatoblast is the protoblast of two very important organs, 

 viz. : the shell-gland and the ventral plate. ^ 



I cannot forbear calling attention again to the wonderful 

 similarity of the cleavages of the first somatoblast in Nereis. 

 {Cf. table on p. 410 of E. B. Wilson's Nereis work.) In Unio 

 the first somatoblast buds forth four small cells, whereas in 

 Nereis it buds forth three small cells only, before its bilateral 

 cleavage ; the place of formation and relative size of the cells 

 is exactly the same as in Unio. One would expect from its 

 original position that it would form the mid-dorsal region in 

 Nereis as in Unio ; and as a matter of fact such seems to be 



1 I use the term " ventral plate " with full knowledge of its significance in 

 annelid embryology. The term is, however, so applicable to this tongue of cells, 

 and the fate of the cells in question is so similar in part to those of the Annelid 

 ventral plate that I feel justified in using the term. 



