160 GEORGE W. TANNREUTHER 
first. The order of their formation is d? (X), c?, b?, a2, the same 
as the first generation. Figure 24 illustrates the twelve-cell 
stage from the upper pole. The cells are somewhat flattened. 
The macromere D is located a little to the left of the median 
longitudinal plane, while X is symmetrical with reference to 
the median axis of the future worm. 
The cleavage spindles of the third generation of ectomeres 
form in an oblique direction. The thirteen-cell stage is reached 
by the formation of d*. The manner in which d? is formed, is 
rather unique when we take into consideration the size and 
position of D with reference to the other macromeres A, B and 
C (figs. 28-25). It is budded off from the outer surface of 
D and takes up a position symmetrical with c*?. The fourteen- 
cell stage is reached by the formation of x! (fig. 27), it is budded 
off from the median right side of X. Its final position is between 
X and C. Figure 28 shows the same stage as the preceding in 
ventral view, turned a little to the observer’s right. 
Immediately after the formation of x! the macromere C buds 
off c?, thus making a distinct fifteen-cell stage (fig. 29). The order 
of formation of the third generation of ectomeres is the same as 
the first and second. Figures 29, 30, 31 and 32 show the fifteen- 
cell stage in dorsal, left, right and ventral views respectively. 
In figure 33, a® and b* are formed and the first generation of ecto- 
meres are preparing to divide; d! and ec! divide first; at the same 
time x? is budded off from X, symmetrical with x!, between D 
and d’, thus making a twenty-cell stage (fig. 34). Figure 35 
is a side view of a twenty-two-cell stage after. a! and b! have 
divided. This figure represents an anterior posterior elongation 
of the embryo, which is a very common occurrence. ‘The cells, 
taken as amass, are very plastic and may assume different shapes. 
This peculiarity is only secondary and has no special significance. 
The cells become more spherical before division and flatten out 
somewhat after the division is complete. 
The twenty-three-cell stage is reached by the formation of x® 
from the upper posterior side of X between ¢! and d! (fig. 36). 
The division of the first generation of ectomeres is unequal 
and radial rather than oblique. From the twenty- to the thirty- 
