170 GEORGE W. TANNREUTHER 
5. THE ENTOBLAST 
The formation of the entoblast in Bdellodrilus represents an 
unusual type of development among the annelid worms. The 
macromeres A, B, C and D, after the formation of d‘, give rise 
to the entire entoderm. D is greatly reduced after the forma- 
tion of d‘. The position of the entomeres is shown in figures 
40 and 42. In figure 40 A, B and C appear rather large, be- 
cause of the flattened condition of the cells. In figure 42 (from 
the ventral pole) the cells are rounded and appear more normal. 
The position of the entomeres and their boundary cells are dis- 
tinectly shown. This figure shows more clearly the bulk of 
the entoderm, when compared with the mass of the entire egg. 
In figure 43 (a twenty-nine-cell stage) A, B and C have divided 
nearly equally. This division is considered by some investi- 
gators as the formation of the fourth generation of micromeres; 
d‘ of the D quadrant has formed earlier. Figures 44 and 45 
(apical pole views) show the upper outer edge of the entodermal 
cells. In figure 46 (the same stage as preceding from the ven- 
tral pole) a very small part of the entodermal and ectodermal 
cells are visible. This figure shows the prominence of the four 
large cells, which later form the ten teloblasts. These four large 
cells, from their position, resemble the four large entomeres, 
which are so prominent in many other forms. These cells 
(X, X, M, M), according to Selensky, share equally in the 
formation of the germ layers, i.e., ectoderm, endoderm and meso- 
derm are produced by each of them. 
In forms like Clepsine, Crepidula and others, at a similar 
or later stage of development, the entomeres are very prominent 
and the ectomeres with the first and second scmatoblasts, form 
a cap of cells on their upper surface. In Bdellodrilus the con- 
ditions are different. The ectomeres and the entodermal cells 
form a cap of cells on the upper anterior surface of X, X, M and 
M. This difference is due to the prominence of the first and 
second somatoblasts, which constitute the greatest bulk of the 
embryo. At about the seventy-cell stage the ectodermal and 
endodermal cells are nearly uniform in size (figs. 47-49). In 
