ADAPTATION IN CLEAVAGE. 



51 



Let us now look at the first four divisions of D, the largest 

 cell of the four-celled stage (Fig. 2). Its first division (Fig. 3) 



Fig. 3. Fig. 4. 



P]Q 2 — Fight-celled stage of Unto coinplanata from the animal pole. 



PiQ ^. — Fighteen-celled stage of Unio complanata from the vegetative pole. The two cells 



ruled with horizontal lines are products, the first somatoblast, d'^ = A', which forms the 



trunk, including the shell-gland and foot of the larva. 



is very unequal, and the smaller product, perhaps not one- 

 tenth of the whole, lies nearer the animal pole ; it is one of the 

 first generation of ectomeres. The second division is likewise 

 unequal (Fig. 4), but here the 

 relations are reversed, for two- 

 thirds, at least, of the substance 

 of the cell passes into the upper 

 product, the first somatoblast, rt'- 

 or A'; the third division is likewise 

 unequal (D and d3, Fig. 4), and 

 this time again the smaller prod- 

 uct is uppermost, forming one of 

 the relatively unimportant third 

 generation of ectomeres. The 

 fourth division, finally, is extremely 

 unequal (Fig. 5), only a minute 

 portion of the cell remaining at 

 the lower pole, while the remain- 

 der forms the second somatoblast, 

 d4 or Af, the proteloblast of th 

 these unequal divisions there is 



Fig. 5. — Thirty-two-celled stage of Uniocom- 

 platiaia from the vegetative pole. The 

 separation of the germ-layers is practically 

 complete in this stage. Products of first 

 somatoblast, d" = X. with horizontal lines : 

 second somatoblast, d* = M (mesoblast 

 cell), with vertical lines-; endoderm cells 

 with both vertical and horizontal lines. 

 K=a-'2 larval mesoblast. 



e mesoderm. In each of 

 a manifest adaptation, the 



