EMBRYOLOGY OF BDELLODRILUS a 
parent cell in the formation of the daughter cells, suggest that 
the apparent rotation process is not mechanical or even pro- 
duced by pressure of the macromeres. The formation of d! 
in figure 10 shows how the process takes place; d! is budded off 
obliquely from the macromere D over the inner posterior edge 
of A and becomes partly imbedded in A. Its final position is 
determined by the direction of the cleavage spindle. This 
characteristic method in the formation of the ectomeres is quite 
a prevalent one. It occurs not only in the eggs of annelids, but 
in those of the molluses and polyclads as well. 
Fourth cleavage: A nine-cell stage is reached in Bdellodrilus 
by the division of the macromere D in an oblique direction. 
Figure 16, an eight-cell stage viewed from the left side, shows 
the position of the macromeres A, B and C with reference to the 
macromere D, before the formation of the ectomere d?. The 
large macromere D contains about two-thirds of the volume of 
the dividing ovum. In preparation for the formation of d?, D 
elongates in an oblique direction at an angle of about forty-five 
degrees to the horizontal plane of the developing embryo. The 
ventral anterior portion of D shifts forward beneath A, B and C 
(fig. 17). After the formation of d?, D takes a position directly 
beneath the first generation of ectomeres, and completely covers 
the inner ends of A, B and C (figs. 19-20). In some instances 
D is shifted more anterior and completely covers the ventral 
surface of the other macromeres (fig. 20); but in most cases, as 
in the nine-cell stage, D occupies the region of the ventral pole, 
directly beneath the first generation of ectomeres (figs. 22-23). 
The formation of d? is shown in figures 16 to 19. The division 
is equal in most cases. When unequal, d? is the larger cell. 
In figure 20, a nine-cell stage turned to the observers left so that 
all the cells are visible, A and B are preparing for the formation 
of a? and b?. In most the succeeding stages d?, the ‘first soma- 
toblast’ will be designated by the capital letter X. It is also the 
first cell of the second generation of ectomeres. The formation 
of a?, b? and c? is shown in figures 20, 21, 22 and 24 in side and top 
views respectively. The second generation of ectomeres, with 
the exception of d? (X), is about the same size as those of the 
