ADAPTA TION IN CLEA VA GE. 49 



what was required of it, and acted accordingly ? Can we deny 

 that the rate of cleavage of this cell is an adaptive feature of 

 the cleavage ? The other cell of the second generation that 

 divides more rapidly than the average rate is a 2 - 2 , or F, the lower 

 product of the first division of a 2 . In the thirty-two-celled 

 stage (Fig. 5) this cell already projects some distance into the 

 segmentation cavity ; after budding off two more small cells at 

 the surface, it withdraws entirely within .the segmentation cav- 

 ity, and divides equally into two cells, which arrange themselves 

 symmetrically on either side of the middle line. From these 

 cells of the larval mesoblast the adductor muscle of the glo- 

 chidium and certain isolated myocytes of the cleavage cavity are 

 derived. Now the adductor muscle is an extremely large organ, 

 of the utmost functional importance to the larva, and the varia- 

 tion in the rate of cleavage of the cells which are to form it 

 appears to be adaptive. 



The third generation of ectomeres possesses the normal num- 

 ber of cells for this stage, but it is interesting to notice that 

 the posterior member, d3, is formed much before the others, 

 owing to the tendency to more rapid division of the basal cell 

 from which it arises. This cell, D, has also undergone another 

 division, in excess of the divisions of the other basal cells, giv- 

 ing rise to the mesoderm proteloblast, d* or M. 



To summarize : groups of cells and even single cells vary 

 greatly in their rates of segmentation. Each acts as though 

 animated by an independent force. Moreover, these differ- 

 ences possess prospective significance, looking forward to the 

 final outcome ; and this phenomenon may fairly be called adap- 

 tation in the rate of cleavage. 



But the objection may be raised : granted that the rate of 

 division varies in these cells, and that those segmenting more 

 rapidly form the earliest functioning parts, is it not possible 

 that the variation is induced from without, and that this deter- 

 mines the subsequent fate ? So far as we know, the only 

 external factors which influence the rate of cleavage are tem- 

 perature, chemical constitution of the medium, and perhaps 

 some other general conditions of the environment, which must 

 act uniformly on the whole egg, and hence cannot be held 



