EMBRYOLOGY OF BDELLODRILUS 213 
trochophore to its approximate or complete disappearance; on 
the other hand, there is a gradual acceleration in the time of 
the trunk development, varying according to the increase in 
the relative size of the first somatoblast or X. Treadwell states 
that the extra amount cf material stored in the macromere D 
is in some way related to the amount of somatic and mesoblastic 
material needed in the future organism. This statement is 
true of the condition that occurs in such annelids as Bdellodrilus 
and Clepsine. 
GENERAL SUMMARY 
The undivided egg of Bdellodrilus philadelphicus is nearly 
oval. Its median longitudinal axis through the region of the 
polar bodies corresponds to the median axis of the future adult. 
The polar bodies occupy the region which later becomes the 
anterior end of the embryo. 
The first cleavage plane is nearly at right angles to the median 
axis of the resulting individual, and divides the egg into two 
very unequal parts. The second cleavage occurs at an angle 
of about forty-five degrees to the first. It divides the smaller 
cell nearly equally and the larger cell very unequally; the larger 
cell divides first. In a four-celled embryo the large cell D is 
posterior, B is anterior, inclined a little to the right; A left and, 
C right. 
The ectoderm is separated from the four macromeres by a 
series of three oblique cleavages. The first generation of ecto- 
meres is formed in a dexiotropic direction. The second genera- 
tion laeotropically and the third in a dexiotropic fashion. 
In the fourth generation of micromeres, d‘ is mesoblastic. 
The other cells of the fourth quartette, together with the four 
macromeres, form the entoderm. The cleavage of the ento- 
dermal cells is carried to the end without delay, in the forma- 
tion of the digestive tract, and the interior of the embryo be- 
comes a solid mass of entodermal cleavage cells, which later 
become differentiated into the epithelial portion of the alimen- 
tary canal. As the core of entodermal cells grows posteriorly, 
