EMBRYOLOGY OF BDELLODRILUS 209 
in. Bdellodrilus is meridional and very unequal. In the two-cell 
stage the larger cell is posterior and the smaller cell anterior. The 
larger cell divides first and very unequally, while the smaller 
cell divides nearly equal (text figs. 1-3 and fig. 5). In the four- 
cell stage D is posterior, C right, A left and B anterior, inclined 
a little to the right. Thus it is very evident that the four-cell 
stage illustrates a difference in the rate of cleavage, a differ- 
ence in the size of the cells, and a difference in the direction of 
the cleavage. The significance of these variations may be em- 
phasized as follows: 
a. Difference in the rate of cleavage of cells 
If we compare a thirty-two-cell stage of Bdellodrilus with 
other forms or perhaps, better, with an ideal ovum, in which 
there is a uniform rate of cleavage in the formation of the cleav- 
age cells, a uniform size and a uniform direction of cleavage, 
a distinct variation occurs as shown in table 5. 
TABLE 5 
CREPIDULA rer Bai | NEREIS ey 
in Soe Pon cane —_s ————— ee a | =i53 | ; 
| 
First generation of ectomeres............ 12 16rs.| 16 8 
Second generation of ectomeres.......... 9 8 8 11 
Third generation of ectomeres............ 5 4 t 4 
“NL TEYETE! CPT OY ee noe a a 0 0 2 
HEEL) DISSE SRP A a eS a ea 4 4 4 ih 
32 32 32 32 
It is evident that in the first generation of ectomeres Bdellodril- 
us departs very far from the ideal condition. The first generation 
contains eight cells instead of sixteen. This means that the 
cells have divided more slowly than in the ideal ovum. In an 
ideal ovum these cells form the prototroch and the entire region 
in front of it, with the apical plate in the center. Jn Bdellodrilus, 
this region is degenerate and no trace of the apical plate appears. 
This indicates an adaptive modification—eight cells instead of 
sixteen—due to a degenerate frontal region. 
In the second generation of ectomeres, the ideal number is 
