18 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
blastomere A, while a5? represents the most dorsal one, the others 
occupying intermediate positions. 
"E 
"x { 
jia P 
nmi 
158 { 
gob 
A3 
[o^ | ai |f oy 
41 uch 
1 p? H 646.2 
ABCD i ait 
08 ql 
OD: 
D3 
1 cell. 2 cells. | 4 cells. | 8 cells. | 16 cells. | 32 cells. 
First Cleavage. 
After the formation of the polar cell the nucleus (Plate 1, Fig. 2) 
returns to the position formerly occupied by the germinal vesicle in the 
longitudinal axis of the egg, lying nearer that end of the ovum in prox- 
imity to which the polar cell was formed (the micromere end). It takes 
such a position that a plane at right angles to the long axis of the egg 
and cutting the polar cell would also cut the nucleus. The distance 
from the centre of the nucleus to the nearer end of the egg is about 
two fifths of the length of the egg. Here two asters appear on oppo- 
site sides of the nucleus, the line joining them being oblique to the 
long axis of the egg. Though I have examined a large number of eggs 
at this stage, in no case have I been able to observe a stage in the 
process of forming the two asters of the first cleavage spindle (Fig. 3) 
from the single aster remaining after the formation of the polar cell 
(Fig. 2). 
Between the two asters a spindle is formed. This lies at first some- 
what oblique to the longitudinal axis of the egg, as shown in Figure 3, 
but before cleavage takes place the spindle swings into coincidence with 
the long axis (Fig. 4). The aster lying at the micromere end of the 
spindle is distinctly smaller than the opposite one (Fig. 3). The nucleus 
becomes divided into two small masses, which move toward opposite 
