212 : GEORGE W. TANNREUTHER 
each proteloblast buds off a small cell, x‘, one to the right of xt 
and the other to the left of x2. Again each proteloblast buds off 
a small cell, x*, on either side of x*. At the next division each 
buds off a small cell, x®, on the ventral anterior edge. Later, 
x7 is budded off from each neuroblast on the ventral side. These 
small cells give rise to the trunk ectoderm and the larger cells 
to the nephridia and nervous system. Here again the cells are 
formed just where they are needed; the smaller on the exterior 
or outer surface while the larger remain within. 
2. ADAPTATION IN THE CLEAVAGE OF OTHER FORMS 
In following the variation of cleavage cells in annelids and 
molluses, special cells can be arranged in a complete series, from 
those of an almost insignificant size to an extremely large cell. 
In following these variations, step by step, we can not fail to 
be convinced that these variations are adaptive to the future 
needs and habits of the larva and of the adult animal. 
In forms with equal cleavage, the first somatoblast gives 
rise to the ectoderm of the trunk region. In Polydorgus, Po- 
darke, Hydroides, Eupomatus and others with equal cleavage, 
d? is the same in size as the cells of the other quadrants. Equal 
cleavage has been offered by some as due to a lack of differentia- 
tion in the early stages but in such forms as Podarke with 
equally cleavage, very early differentiation occurs, and the 
prominence of these early functioning parts varies according to 
the size of the initial cell from which they are formed. 
In forms with unequal cleavage, the first somatoblast differs 
in size from the remaining members of the same quartette. 
Beginning with Amphitrite, the relative size of d? increases suc- 
cessively in Chaetopterus, Arenicola, Nereis, Capitella, Aricia, 
Spio, Clepsine and Bdellodrilus. Those forms with equal cleav- 
age pass through a distinct trochophore stage and are charac- 
terized by an almost equatorial prototroch, a very large exum- 
brella, and with a very slow trunk development. In those with 
unequal cleavage, especially in the second generation of ecto- 
meres, there is a gradual decrease in the prominence of the 
