No. 3.] ZHE EMBRYOLOGY OF THE EARTHWORM. 399 
in Rhynchelmis, where they arise in a manner closely similar 
to that studied by Whitman in Clepszne (Nos. 45, 48). 
After the formation of the first pair of secondary mesoblastic 
cells others are formed by successive divisions of the primary 
mesoblasts, which in the meantime gradually sink below the 
surface of the blastula and are finally quite included in the 
cleavage cavity (Figs. 24-30). Thus arise two parallel rows 
of mesoblastic cells which lie in the cavity of the still spherical 
blastula and define the antero-posterior axis of the embryo. 
Posteriorly the two rows terminate in the primary mesoblasts ; 
anteriorly they join one another in the middle line. The 
blastula retains its spherical form until five or six pair of second--. 
ary mesoblastic cells have become formed and the primary 
mesoblasts have become entirely enclosed. 
This period is marked by the appearance of a large pore 
(“cleavage-pore’’) opening from the blastocoel to the outside 
at the lower or entoblastic pole—a remarkable phenomenon 
already observed by Kleinenberg. This pore is formed by the 
separation of the larger cells during the active periods of division, 
now very marked, which alternate with equally marked quies- 
cent stages. I have never found a trace of the pore during the 
quiescent periods, but it is certainly present in more than one 
of the active periods. (Compare Figs. 22 and 26.) It seems 
probable therefore that it appears in several successive active 
periods, closing in the succeeding quiescent periods. The only 
explanation of it that occurs to me is that it may serve as a 
channel for the expulsion or entrance of liquid during the active 
stage, so as to avoid undue pressure on the cells during their 
change of form. 
IV. GASTRULATION. 
A modified embolic invagination takes place in the course 
usually of the third day, though the rate of development varies 
so greatly with the temperature and differs so widely among 
individuals that no general statement of the time has any value. 
The spherical blastula becomes flattened and slightly elongated 
(Figs. 31, 32), finally assuming the form of an oval plate with 
rounded edges (Figs. 33, 34, 39, 40). During this general 
change of form the cells undergo striking alterations. The 
larger cells of the lower pole become clearer, assume a columnar 
