48 BIOLOGICAL LECTURES. 



It is plain that as regards the relative rate of cleavage of the 

 constituent cells, the egg of Unio departs very far from the ideal 

 condition. Let us attempt to analyze this and see if these vari- 

 ations in the rate of cleavage are adaptive or not. The first 

 thing we notice is that the number of the first generation of 

 ectomeres is ten instead of sixteen ; this means that these cells 

 have divided more slowly than the average rate. Now in most 

 mollusks and annelids they form the prototroch and the entire 

 region in front of it, with the apical plate in the centre. But 

 in Unio this region is degenerate, and I have never been able 

 to find any trace of prototroch or apical sense-organ. It is 

 plain, then, that the slow rate of cleavage is anticipatory of 

 this, and is, in this sense, an adaptive modification. But why 

 ten cells in place of eight ? This means that two of the cells, 

 c l and d 1 , of this group have divided once oftener than their 

 fellows. These cells occupy the posterior portion of the upper 

 hemisphere, and later enter into the formation of the head- 

 vesicle, and I believe that their more rapid cleavage is an early 

 indication of their subsequent fate. 



In the second generation of ectomeres we have a very dif- 

 ferent set of conditions as to number ; the ideal number of cells 

 of this generation in the thirty-two-celled stage is eight, but in 

 Unio thirteen. This increase above the ideal number is due 

 almost entirely to the rapidly succeeding divisions of one cell 

 of this generation, d 2 , but in part also to another, a 2 . The 

 other two cells, the anterior and the right members of the 

 quartet, fall in with the ideal scheme, each having divided 

 once. In this stage d 2 has broken up into six cells, a 2 into 

 three. Is there anything adaptive in this hastening of the 

 process of cleavage ? d 2 is the cell from which the entire ecto- 

 derm of the trunk region is formed. The most characteristic 

 thing about this region in the embryo of Unio is the relatively 

 enormous size and very early formation of the shell-gland, and 

 we have seen that the shell of the glochidium is of very special 

 importance to it, not only as an organ of defence, but also of 

 offence. Is it not wonderful that this important part should 

 be represented, not only by the largest, but also by the most 

 rapidly segmenting cell in the embryo, as though the egg knew 



