No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 31 



oblique cleavages, the first of which is dexiotropic, the second 

 leiotropic, and the third dexiotropic again. The next division 

 (/.(?., the fourth) of the posterior macromere separates the pro- 

 teloblasts of the mesoderm. 



The first generation of ectomeres divides very slowly. Its 

 cells are destined for the anterior end of the future larva. 



The second generation of ectomeres is remarkable for being 

 composed of the largest cells in the embryo. The posterior 

 member d^ or X is the protoblast of the shell-gland and 

 ventral plate. The larger part of the left member (i.e., a--^ or 

 Y) forms that portion of the mesoblast which I have called 

 the larval mesoblast. <«^', b^, and c^ enter into the formation 

 of the larval mantle. The third generation of ectomeres 

 probably does the same. 



The entomeres are small. They undergo division before 

 invagination. The resulting archenteron is extremely small, 

 but it invaginates before the shell-gland ; thus, though rudi- 

 mentary, its formation is not delayed. 



The shell-gland is a voluminous structure formed from a 

 plate of large columnar cells, which are derived from repeated 

 divisions of X. 



The first bilateral cleavage is that of the first somatoblast ; 

 the second, that of the second somatoblast. 



The primary mesoblasts enter the segmentation cavity and 

 lie just behind the archenteron, thus in the angle which the 

 latter makes with the shell-gland. Their divisions are typically 

 teloblastic. It should be said that, before they enter the 

 segmentation cavity, each buds off a small cell at the surface. 

 The larval mesoblast conforms to the bilaterality of the 

 embryo after entering the segmentation cavity. Its divisions 

 are not teloblastic. 



The table of cleavages which follows gives, in some detail, 

 the order of the divisions and the destiny of the blastomeres. 

 The stages indicated by columns separated by continuous lines 

 represent, more or less nearly, natural periods of rest of the 

 entire ovum. The vertical columns united by dotted lines 

 indicate more or less synchronous divisions. This method was 

 adopted to show clearly the very irregular course of the 



