No. I.] THE EMBRYOLOGY OF CREPIDULA. 89 



the basal and middle cells of the arm, Figs. 65-68, 70, and 

 Diagram 10. At nearly the same time the anterior interme- 

 diate cells divide into upper and lower portions, which lie on 

 each side of the anterior arm. Fig. 56 and Diagram 10. 



Next, as shown in Fig. 53, the two cells (proximal and distal 

 tip cells) derived from the tip cell of the posterior arm divide 

 in the direction of the long axis of the arm, forming four cells 

 which have come from the single original tip cell. These four 

 cells lie in line with the two proximal cells of the posterior arm, 

 so that now the entire arm consists of a linear series of six cells. 



Finally, the last stage in which I have been able to recog- 

 nize the entire cross is shown in Fig. 56 and Diagram 11. At 

 this stage the two tip cells which lie at the ends of the right 

 and the left arms divide across the axis of the arms, so that 

 there are four tip cells at the end of each arm. Unlike the four 

 tip cells of the posterior arm, these do not lie in the long axis 

 of the arm, but across it. About the same time all the middle 

 and intermediate cells of the transverse arms divide radially, 

 so that there come to be two right and two left middle cells, 

 and two right and two left inter- 

 mediate cells in each arm, one 

 of the two being proximal, the 

 other distal in each case. 



The fate of the tip cells of the 

 anterior arm is very uncertain. 

 I have not been able to trace 

 them satisfactorily beyond the 

 stage shown in Fig. 56, when 

 they are still very small and in- 

 significant. However, I believe 



that 171 C. plana they are crowded diagram h. — The first quartette in Crepi- 



entirely out of the layer of ecto- '^"l'^' ^^T"?, *^ '"''"■ •^=*°'^ °^ '^^ "°=^ 



■^ •' -'J and turret cells. 



blast cells, and that they are 



thrown wholly away} Fig. 71, 2b''-^ shows the two anterior tip 

 cells very plainly. Even here, however, they show important 

 changes in position and structure ; they project above the level 

 of the surrounding ectoblast cells, and their nuclei have no defi- 

 nite boundaries, while the chromatin seems to be dissolving and 



1 See Note p. 204. 



