50 BIOLOGICAL LECTURES. 



accountable for differences in the rate of cleavage of constitu- 

 ent elements of the same egg. In addition to these factors, 

 the presence of a larger proportion of yolk in certain cells may 

 determine a slower rate of cleavage. But in the egg of Unio 

 the yolk is distributed with perfect uniformity among the cells, 

 so that each possesses practically the same relative quantity. 

 Is it not, then, the absolute amount of yolk that determines 

 the rate ? No, unless one is willing to adopt the hypothesis 

 that yolk tends to hasten the cleavage, for it is the largest cell 

 in the egg that segments most rapidly. However, there is no 

 constant relation between size and rate of cleavage, so even 

 such an inverted hypothesis will not avail. There is, in fact, 

 no escape from the conclusion that the factors determining 

 differences in the rate of cleavage of the separate cells or defi- 

 nite cell-groups are of the same nature as those that deter- 

 mine differences between ova of different species, viz., purely 

 constitutional causes. In a very real sense, each cell in the 

 segmenting egg of Unio acts, in respect to rate of cleavage, 

 like an ovum in ovo. 1 



b. Variations in Size. Although difference in size of 

 daughter-cells in normal cleavage generally indicates difference 

 in quality or constitution, it is not always safe to conclude that 

 daughter-cells of the same size necessarily possess the same 

 constitution. In other words, a division may be differential, 

 even though the resulting cells are of the same bulk. The 

 adaptiveness of determinate cleavage is illustrated in a man- 

 ner even more striking than in differences of rate, by the 

 inequality of certain divisions. I believe that it can be shown 

 that the relative sizes of the cells in the early cleavages of the 

 egg of Unio is adapted to the size and time of development of 

 the larval organs. The first two cleavages are unequal with the 

 result that two cells of the four-celled stage are larger than the 

 two other equal ones. The largest of all contains the material 

 of the two somatoblasts, and the next largest the material of 

 the larval mesoblast. 



1 I believe that the entire egg is an undivided organism, and not a heap of equiva- 

 lent cells, each of which may occupy any position in the whole, as Hertwig and 

 Driesch have maintained. The above statement must be understood in this sense. 



