ADAPTATION IN CLEAVAGE. 



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. 



FIG. 3. Eight-celled stage of Unto complanata from the animal pole. 

 FIG. 4. Eighteen-celled stage of Unto complanata from the vegetative pole. The two cells 



ruled with horizontal lines are products, the first somatoblast, d 2 = X, 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, d- 

 or X\ 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 FIG. 5 . Thiny-two-ceiied stage of uniocom- 



,1 T < 11 i planata from the vegetative pole. The 



tOUrth diVlSlOn, finally, IS extremely separation of the germ-layers is practically 



unequal (Fig. 5), only a minute complete in this stage ' Products of first 

 portion of the cell remaining at 



the lower pole, While the remain- with both vertical and horizontal lines. 



somatoblast, d z =- X, with horizontal lines ; 

 second somatoblast, d* = M (mesoblast 

 cell), with vertical lines ; endoderm cells 



=a 2 ' 2 larval mesoblast. 



der forms the second somatoblast, 



d4 or M, the proteloblast of the mesoderm. In each of 



these unequal divisions there is a manifest adaptation, the 



