192 GEORGE W. TANNREUTHER 
Fig. 23 Nine-cell stage of Clepsine, upper pole (after Whitman). 
Fig. 24 Nine-cell stage of Unio from behind (after Lillie). 
Fig. 25 Nine-cell stage of Dinophilus, left side (after Nelson). 
Fig. 26 Nine-cell stage of Bdellodrilus, left side. 
not occur in this manner in Dinophilus, Bdellodrilus or even in 
Nereis. The fourth division in the above three forms is equal 
and bilateral, while in Unio the fifth cleavage of X is the first 
bilateral division and forms X, X right and left. Next X, X 
on either side divides nearly equally and gives rise to the shell 
gland. These four cells might be regarded as the posterior telo- 
blasts, which occur in other forms, as in Nereis and Dinophilus. 
In Bdellodrilus X is formed by an equal division of the macro- 
mere D (text fig. 26), and takes a median posterior position. 
First x! is budded off from X to the right, posterior toC. Then 
x? is budded off to the left, symmetrical with x' and posterior to 
d?. Next x? is formed from the median dorsal anterior edge of 
X, between d! and c'. Now the first bilateral division of X takes 
place and forms the proteloblasts X, X, right and left. Each 
of the proteloblasts bud off x‘, x® and x® respectively. At the 
