EMBRYOLOGY OF BDELLODRILUS PAI 
The largest cell at the four-cell stage is D. Its first division is 
very unequal, and the smaller cell is less than one-tenth the size 
of the larger. It is the first ectomere of the first generation 
formed. The second division, in most instances, is equal; when 
unequal, the largest cell passes into the upper product, and 
forms d?, the first somatoblast. The third division is unequal, 
and d*, the smaller product, is again uppermost; and finally, 
the fourth division is very unequal and only a small portion re- 
mains as the macromere D. The greater bulk, d‘, becomes the 
second somatoblast. In each of the above instances the larger 
cells form a large part of the embryo and the adult, while the 
smaller cells, in-every instance, form a very insignificant portion. 
The unequal division in each instance is evidently adaptive, 
for the great bulk of the material passes into the two somatoblasts 
and gives rise to the muscular, nervous and excretory systems. 
c. Variation in the direction of cleavage 
Here only some of the special cleavages will be emphasized. 
The first division of the second somatoblast is equal, and each 
part forms equal parts of the mesoderm. Next, each primary 
mesoblast buds off five or six small cells beneath the first quar- 
tette of ectomeres. These small cells remain quiescent for a 
considerable period and later give rise to the dorsal mesoderm. 
Immediately after these small cells are formed, the mesoblast 
bands are begun by a forward proliferation of cells from the an- 
terior face of the primary mesoblasts. The plane of division is 
at right angles to that of the small cells. These bands extend 
forward between the ectoderm and the entoderm, and at the 
same time the entodermal cells extend posteriorly between the 
mesoblast bands and the group of small cells, thus separating 
the primary and secondary mesoderm. Here the direction of 
the cleavages place the .cells where they are later used in the 
formation of some special part, adapted for that particular 
region. . 
The first somatoblast buds off x! to the right, x? to the left, 
and x? median dorsal anterior. X now divides equally and 
