EMBRYOLOGY OF BDELLODRILUS 163 
the establishment of the germ layers, the bilateral division of 
the ‘first and second somatoblasts’ occurs. The bilateral divi- 
sion of the ‘second somatoblast’ usually precedes that of the 
first; occasionally they divide simultaneously. 
c. The bilateral period of cleavage: twenty-five-cell stage 
The first bilateral cleavages occur in the first and second 
somatoblasts (text fig. 7 and figs. 37-43). The small super- 
ficial cells of the lower pole are derived from the second and 
third generation of ectomeres and from the derivatives of X. 
The arrangement of these cells with reference to the blastopore 
is shown in figure 42. The entomeres A, B, C and D are partly 
grown over by the other cells and the open space becomes the 
blastopore. It is bounded anteriorly and laterally by small 
cells from the second and third quartettes, and posteriorly by 
the primary mesoblasts M,M. Its hinder lip, which is formed 
by the primary mesoblasts, lies anterior to the center of the 
lower pole. The closure of the blastopore takes place by a con- 
vergence of the cells from all sides. The principal growth of 
cells is from in front backwards, formed by the derivatives of 
the second and third generation of ectomeres (figs. 42, 48, 51). 
The entomeres now divide very rapidly and the cells soon be- 
come smaller than those of the ectomeres, which grow over them 
(figs. 43, 51). 
3. THE FIRST SOMATOBLAST 
The history of the ‘first somatoblast’ in Bdellodrilus is of 
considerable interest when considered from the standpoint of 
its origin and its derivatives. When first formed from the 
posterior macromere D, it contains one-third of the entire bulk 
of the developing embryo. As already described, it first buds 
off the small cell x' on the right, x? symmetrically on the left 
and a third cell, x°, on the median posterior upper side. These 
three small cells are symmetrical with reference to the median 
longitudinal axis. The fourth cleavage divides the somato- 
