10 GENERAL DEVELOPMENT OF THE EMBRYO. 



pose in fact that the endoderm cells of this stage are amoeboid 

 and capable of independent 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, and apparently, 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 aggregation 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 difi'erent 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). 



The ectoderm now grows round the endoderm cells and 

 entirely surrounds them, excepting at one point. At this 

 point, which is opposite to that on which the ectoderm cap 



