No. I.] THE EMBRYOLOGY OF CREPIDULA. 133 



from seven cells of the anterior arm of the cross, and that the 

 terminal cells of this arm are probably thrown away.i There- 

 fore the cells lying immediately beyond the anterior arm are 

 the ones from which this median ventral portion of the first 

 velar row comes. These cells may be traced back to a single 

 one, 2b'-^-^-% Figs. 46, 47, which lies at the end of the anterior 

 arm. In this same position two cells are found a little later, 

 Fig. 50, which have evidently come from this single cell by 

 equal cleavage ; these cells are therefore 2b'-^-^'' and 2b'-^=''-='. 

 At a later stage they increase to four. Figs. 56, 71, and finally 

 to six in Fig. ']2. 



The portion of the first velar row lateral to these six cells is 

 evidently derived from the anterior turret cells, la^ and ib^. 

 These cells are shown undivided in Fig. 49, while in Fig. 50 

 they have divided bilaterally into the cells la^' and ia^% ib^' 

 and ib^-^. They are characterized by having clear protoplasm 

 and large nuclei, and can be recognized for a considerable 

 period lying just in front of the terminal cells of the right and 

 left arms of the cross, and on each side of the median velar 

 cells. Figs. 50-56. 



The earliest figure which shows the protoblasts of the first 

 velar row in position is Fig. 50 ; there are here two median 

 and four turret cells, forming a series of six cells surrounding 

 the cross on its anterior side, and extending from tip to tip 

 of the transverse arms. In Fig. 56 they are shown increased 

 to eight by the bilateral division of the median cells, and in 

 Fig. 62 to ten or twelve. The terminal cells of the transverse 

 arms of the cross divide, forming a row of four cells across the 

 end of each arm. Fig. 56, and it is probable that these also 

 must be added to the velar cells already described, making 

 in Fig. 50 ten velar cells, in Fig. 56 sixteen, and in Fig. 62 at 

 least eighteen or twenty. These cells belong to the first velar 

 row only, and they extend from the mid-ventral line about two- 

 thirds of the way around toward the mid-dorsal line. During 

 all this time the posterior turret cells remain undivided, and 

 the velar row ends dorsally against these cells. 



It is probable that the mid-ventral portion of the second 

 velar row, V^, is derived from the cell which I have identified 



1 See Note p. 204. 



