EMBRYOLOGY OF BDELLODRILUS 193 
next division each proteloblast divides nearly equally, and gives 
rise to X™, neuroblast, and X, nephroblast, on each side of 
the median axis of the embryo. Next each nephroblast divides 
nearly equally and produces X‘ and X‘). Now a very inter- 
esting thing happens; either X° or X® divides (but never both 
in the same egg) and produces the three nephroblasts on each 
side, which are designated as X, X‘? and X™. In Clepsine 
only two of these teloblasts are concerned in the formation of 
the nephridia. The lateral teloblasts, as stated above, are prob- 
ably ectodermal. 
These four forms unquestionably show that there is a marked 
similarity in the cleavage of the ‘first somatoblast,’ not only in 
widely different individuals in the same group, but in individuals 
of widely separated groups. This comparison could be extended 
to other groups or forms, but the above will suffice for our 
purpose. 
2. THE SECOND SOMATOBLAST 
In Clepsine, the ‘second somatoblast’ has rather a unique 
origin. It is formed at about the twelve-cell stage. D, after 
the formation of X, becomes directly the ‘second somatoblast’ 
or M. (These cells are differently designated by Whitman; D 
is represented by x, X by x! and the mesoblasts by x and xy.) 
M divides nearly equally and produces the right and left meso- 
blasts, from which the inner stratum of the germ bands is formed. 
In Bdellodrilus, M is formed by a very unequal division of the 
macromere D, at the twenty-four-cell stage (fig. 85). The 
larger cell or M is formed in front of X. It is inclined a little 
to the left of the median axis. The first division of M is equal, 
producing the mesoblasts, one on either side. These primary 
mesoblasts now bud off a number of small cells, directly be- 
neath d' and ec! (figs. 86-87). It is very difficult to make out 
the exact number of these small cells, since they are not visible 
externally. There are at least twelve formed, six on either 
side from each mesoblast. After this small group of secondary 
mesodermal cells are formed, the mesoblasts M, M, give rise to 
