EMBRYOLOGY OF BDELLODRILUS 161 
cell stage several types of cleavage are present; oblique, radial 
and bilateral. This period of variable cleavage will be desig- 
nated as the transitional period. 
b. The transitional period of cleavage: twenty- to thirty-cell stage 
After the formation of x® there is a short inactive period and 
in many of the developing embryos, the cleavage furrows become 
very indistinct. Cleavage is again initiated by the formation 
of d‘ from the large macromere D. The cleavage is oblique and 
very unequal (text fig. 5 and fig. 85). 
The smaller cell is almost completely hidden when first formed. 
It is budded off directly between A and B, near the ventral 
anterior surface (fig. 37). The smaller cell persists as D (ento- 
mere) and the larger cell d‘ or M becomes the ‘second somato- 
blast.’ After the formation of M the entire endoderm is 
contained within the entomeres A, B, C and D (figs. 37-88). 
The germ layers are now distinctly separated and the em- 
bryo at this stage of development is composed of twenty-four 
cells (text figs. 6-7). Nereis at the same period of differ- 
entiation, shows thirty-eight cells. Unio (Lillie) at the time 
of the separation of the germ layers contains thirty-two cells. 
This difference is due, in case of Bdellodrilus to the lagging of 
division in the cells of the upper pole. 
The composition of the embryo at the twenty-four-cell stage 
is as follows: 
Bmbomeresa.2..7 2.22. 0 Ney) Bi Orbe Be iy ete ae RRS Beware RAC <2 Sd 4 
Hetomeress.- 4454.0. Orimsincenenatlom:. ....2 2% Sed) oes ene 8 
Hetomeres ye 22.42) S (Olt MSCOMCl mys HMOle se nebo ancanacdodesodes 4 
Befommeresie x 4.211..." Op aindeeenerAtlons... 0.02 s4e Se eee eee 4 
Teron ul Mets ered pacer Mee al LL meee ar Res: HA is says dad in eee eee ene 1 
EUCA eS OnIALOOLAStIGeRUVATI VES: «... as. os. id ste saat hese oaeinelae ees 3 
24 
Many of the cells during the transitional period have a defin- 
ite shape and if separated from the cell complex, they could 
be readily recognized. The embryo at this stage of develop- 
ment is somewhat spherical (figs. 36-38). Immediately after 
