51 
* the separation of a new micromere, y, from the macromere, a , 
at the growing edge of the ectoderm. 
Figure 26 is a view of the same egg as Figures 22, 23 and 
24, but fifty-five minutes later than Figure 24. The ectoderm 
■ cells are now much smaller and more numerous, and the 
macromere is almost covered by them. At the growing edge 
. <7, a new micromere is separating from the macromere, and 
there are now a number of small cells on the median line 
anterior to the polar globule. In dead eggs at this stage a 
transparent cavity separated the inner surface of the layer of 
ectoderm from the macromere, but this space does not appear 
to be normal, and the macromere seems, in living eggs, to be 
in contact with the outer layer, and there is no indication of 
a segmentation cavity. In many respects the segmentation 
of the oyster egg is very similar to that of the egg of Unio, 
as described by Rabl, but in Unio the segmentation cavity 
is present at a much earlier stage than this. 
From this point on I made no attempt to trace the changes 
of individual eggs, but made sketches of new stages as I 
found them. The stages which are figured here are by no 
means all which were observed and sketched ; and I found 
a number of embryos intermediate between nearly all the 
••stages which were reproduced, so that my series was much 
more complete than the series of figures. 
Figure 27 is a surface view of the left side of an embryo 
Twenty-seven hours after impregnation, and Figure 28 is an 
optical Section of the same embryo. The outline of the body 
has undergone considerable change, and the longest axis is 
now the axis which runs from the polar globule to the poste- 
rior end, and the vertical axis, which was the longest during 
the earlier stages, is now the shortest. In a view from above 
or below the outline is elliptical. The optical section, Fig- 
ure 28, shows that the macromere is now divided into two 
large spherules, en, which are almost entirely covered by the 
ectoderm, e c, except over a small area on the dorsal surface. 
The polar globule is now separated from the anterior edge of 
the ectoderm by four cells, which are smaller than those at 
tthe opposite or growing edge. Figure 29, is a view of the 
