No. I.] THE EMBRYOLOGY OF CREPIDULA. 131 



the anterior side, Figs. 49-55. But, though the divisions are 

 slow, the cell growth is rapid, in consequence of which the 

 cells posterior to the transverse arms of the cross become 

 enormous in size. The cells on the anterior side of the ^g^ 

 divide rapidly, but the total growth is less than that of the cells 

 on the posterior side ; consequently the whole apical pole is 

 shifted forward. Figs. 49-55. 



Afterwards, at the extreme posterior end of the embryo, cell 

 divisions begin, and proceed so rapidly that in a very short time 

 there are more ectoderm cells on the posterior than on the 

 anterior side. Fig. 64. The region of greatest activity lies just 

 ventral to the future shell gland, and almost immediately over 

 the mesoblastic teloblasts. Radiating from this region are 

 more or less regular rows of cells. Figs. 64, 65, which are par- 

 ticularly well marked on the ventral surface. I have not been 

 able to identify constantly any ectoblastic teloblasts, though in 

 many eggs there are three or four large ectoblast cells lying 

 between the mesoblastic teloblasts, from which many of the 

 cell rows radiate. Two of these cells are shown at the extreme 

 end of the embryo in Fig. 65. They are large cells with clear 

 protoplasm, and from their position and character I believe 

 that later they become the ciliated anal cells, which are shown 

 in Figs, ^i, 95. 



The cell rows mentioned are much more pronounced in some 

 eggs than in others. In a few cases they seem to cover the 

 whole posterior end of the embryo, though in general they could 

 be distinguished only on the ventral side. Here they run for- 

 ward almost to the mouth as a series of branching, irregular 

 rows. Fig. 65, and include the whole region which ultimately 

 becomes the foot. 



The cells from which these rows radiate lie on the mid line 

 between the mesoblastic teloblasts, and must therefore be 

 descended from the cell 2d. This cell also gives rise to a pos- 

 terior growing-point in the annelids and in Unio, from which, in 

 the case of the former, the ectoblast of the trunk is largely, 

 perhaps entirely, derived, while in the latter the shell gland and 

 foot are formed from the derivatives of this cell. The shell 

 gland and foot in Crepidula are evidently formed from this 



