210 GEORGE W. TANNREUTHER 
eight, while in Bdellodrilus it is eleven. This increase above the 
ideal is due entirely tothe rapid succeeding divisions of one cell— 
d?, the first somatoblast. The other cells of the quartette have 
not divided, while d? has given rise to three new cells. Does the 
behavior of d? suggest any significance, or is it adaptive? From 
d? the ectoderm of the trunk region, the nephridia and the entire 
nervous system is derived; d? is not only the largest but the most 
actively dividing cell of the entire embryo; hence its rate of 
cleavage is well adapted to its resulting formations. _ 
The number of ectomeres in the third generation is the same 
as in an ideal ovum; d? however is often formed before a?*or b? 
of the second generation. This interesting phenomenon is due 
to the tendency of the basal cell, D, and its derivatives to divide 
more rapidly than those of A, B or C. The differences in the 
rate of cleavage in the first, second and third generation of ecto- 
meres, no doubt possess prospective significance, looking for- 
ward to the definitive parts. This may fairly be called adapta- 
tion in the rate of cleavage. 
In Bdellodrilus the more rapidly dividing cells do not form the 
first functioning parts. The cells of the first quartette are 
the first to function, in the production of cilia for the movement 
of the embryo. The variation in the rate of cleavage is not 
due to the varying conditions of the media, or the dividing 
ovum would be uniformly affected, as a whole. Nor is it due to 
the size of the individual cells, as the largest cells divide more 
rapidly. At the thirty-two-cell stage the ideal ovum contains 
four entodermal cells, while in Bdellodrilus there are seven. 
Here the cleavage is carried to the end without any resting 
stage of the four basal cells. This is due to the fact that the 
larva develops very rapidly and the entodermal cells must keep 
pace with the rapid development in order to reach their final 
position, Just where they are needed. 
b. Variation in the size of cells 
The relative sizes of the cells in the early cleavage of the 
eggs of Bdellodrilus are adapted to the later developing parts. 
