458 
ADAM SEDGWICK. 
The endoderm cells at this stage — and I have no doubt this 
is the case in other stages, but in this case the fact can be 
clearly seen — are branched, and the branches of adjoining cells 
in some cases anastomose. One must suppose in fact that the 
endoderm cells of this stage are amoeboid and capable of inde- 
pendent movement, in order to account for the changes which 
now take place. 
In fig. 9 I have had drawn an embryo of this stage as an 
opaque object, with reflected light. The drawing shows clearly 
the mosaic of ectoderm cells, which in this light seem to be 
composed of a brilliant white substance with a central dark 
area. 
The endoderm cells now begin to draw together towards 
the centre of the egg, and come to lie directly beneath the 
ectoderm mosaic, which rests upon them like a cap. I have 
had various stages of this process drawn in figs. 10 — 14. 
This change can only be explained as being due to an 
active movement of the endoderm cells, which travel from 
all parts of the egg towards the centre, where they aggregate 
in masses which gradually unite with one another, forming 
at first a ring and then uniting further until they form one 
more or less spherical solid mass of cells on which the 
ectoderm mosaic rests like a cap. Fig. 15 shows a side view 
of an embryo at this stage, in which this process of aggre- 
gation of the endoderm cells is completed. Fig. 15 is drawn 
from a preserved embryo made transparent by turpentine. 
The nucleus of the ectoderm cells, which has been con- 
spicuous in all these stages by its transparency and freedom 
from granulations, presents quite a different appearance in 
embryos which have been treated with reagents. In the latter 
case instead of a central transparency, we find a central mass 
of dark granules, which are much more marked than the 
granules of the body of the cell. Further, it should be 
pointed out that, in the latter stages, the granulation of the 
ectoderm cells is a much less marked feature [vide figures), 
and that the boundary between the ectoderm cells becomes 
less distinct ( vide especially fig. 13). 
