No. 2.] CLEAVAGE OF A RE NICOLA CRISTATA. 



77 



M-- 



cell being given off ventrally and somewhat laterally. In each 

 succeeding division, however, the direction of the spindle 

 changes, the daughter cell arising each time more toward the 

 upper pole, until finally at the stage 

 of the blastopore closure the spin- 

 dles of the mesoblasts are nearly 

 longitudinal. Thus the main axis 

 of the mesoblast band has changed 

 by nearly 90, but cells given off 

 do not change their own position. 

 Consequently, the anterior ends 

 of the bands are curved ventrally. 

 The position of the right meso- 

 blast and its latest product are shown in Fig. 9, M i and 

 M 4, while the rest of the shaded area shows approximately 

 the extent of that portion of the right mesoblast band already 

 formed. At this stage the change in direction is about half 

 completed. Fig. 1 1 shows the relations of the layers in optical 

 section just before closure of the blastopore, as seen from the 



ventral side and slightly anteriorly. The 

 ends of the mesoblast bands (M) are seen 

 on each side of the endodermic column. 



As stated above, the sixteen primary 

 trochoblasts (Fig. 6, tr I) constitute the first 

 indication of the prototroch. Later these 

 are supplemented by nine cells from the 

 second quartet (Fig. 7, tr II), three from 

 each quadrant, except the dorsal, where a 

 gap remains until about the time of the 

 blastopore-closure. Fig. 8 shows the cells 

 of the prototroch of the right side, the 

 three secondary trochoblasts being lettered 

 tr II. All the trochal cells elongate greatly, 

 and finally the dorsal gap is closed, but 

 before its closure four cells arising from 

 the first quartet of ectomeres pass through it and are shut 

 off by its closure from all connection with the pretrocha] 

 region. 



Par 



FIG. 12. 





I I R A 



