No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 47 



the evagination is completed the shell appears in the form of 

 a very delicate, transparent, cap-shaped cuticle, covering the 

 whole of the shell-gland area, and which is derived from the 

 above-mentioned refringent secretion of the shell-gland. 



As has often been noted, the shell is not at first bivalve ; 

 nor is it so when the gland has evaginated, and the dorsal line 

 begins to undergo lateral compression (Figs. 68 to 71). The 

 cuticular plate is merely bent over the dorsal surface, and 

 adheres on each side to cells which represent the border of the 

 evaginated gland, and wliich are sharply marked off from their 

 neighbors (PI. V, Figs. 75 and ^6). The limits of the shell- 

 gland are thus the limits of the shell. It will be noticed that 

 the nuclei of the cells to which the margin of the shell is 

 attached lie very near the surface, from which I conclude that 

 they play an active part in the secretion and growth of the 

 shell. It is not until some time later that the hinge makes its 

 appearance. 



The cells lying immediately beneath the shell become pro- 

 gressively thinner in proportion as the surface covered by the 

 shell increases. Finally they become reduced to a mere film 

 of protoplasm in which the nuclei produce swellings ( PI. V, 

 Fig- 75 ; f 1- VI, Figs, ^y and 91, etc). The growth of the larval 

 shell takes place wholly from the cells which lie along its 

 margin. As the cells which lie beneath the shell thin out, 

 this region of the embryo becomes perfectly transparent (PI. V, 

 Fig. 79). 



Great changes in the form of the embryo take place at this 

 time. The general character of the changes in question can be 

 seen by a comparison, in serial order, of the figures from 64 

 to 79. The most important elements of the change in form 

 are two: On the one hand, the expansion of the shell-bearing 

 region and the accompanying bilateral compression {cf. Figs. 62 

 and 68, with Figs. ^6 and 80); and, on the other hand, the 

 expansion of the anterior portion of the embryo; that is to say, 

 the portion in front of the blastopore and of the shell-gland. 

 (Compare this region in Figs. 66, 69, and 79.) 



About midway between the blastopore and the cells of the 

 head vesicle {h.v) a slight depression becomes visible at the 



