BY H. L. KESTEVKN. 681 



Tlic rearniugement of the eudo-mesoderm is complex, and eirectetl 

 so quickly that I can do little more than describe the result; but, 

 tirst, let me say that the undifferentiated ectoderm-cells have be- 

 come very thin and transparent, and they certainly overlie the large 

 mesoderm-cells presently to be described; but, as already stated, 

 I am unable to say whether the blastopore closes, though strongly 

 inclined to say it does. 



The first result of the differentiation of the endo-mesoderm 

 appears at the iJosterior end ; here, five cells on each side and three 

 at the end, which form, as it were, a belt on a level with the mid- 

 line, and lying immediately beneath the ectoderm, increase in size 

 and take a deep stain from hsematoxylin. Of the lateral cells, the 

 fourth, on each side, counting from the front, is larger than the 

 others, and these decrease in size on each side of it (Fig. 42). 



Lying deeper in the embryo, dorsal to the belt just mentioned, 

 and directly dorsal to the large cell, but nearer the sagittal plane, 

 tliere is, on each side, another large cell, which differs from those 

 composing the belt, in staining a very little more lightly (Fig. 

 42,g). 



Meanwhile, certain of the cells of the core placed towards the 

 posterior end, have grown in size, apparently at the expense of 

 their neighbours; they have collected uj^ the fat globules, and ap- 

 Ijarently aggregated to themselves the whole of them. 



Tliese cells take practically no stain; their ultimate position is 

 shown in Figs. 45 and 46. As a reference to those figures will show, 

 these cells are of large size and not numerous. They are so 

 arranged that, did they not touch at their inner ends, they would 

 bound a cavity. A few similar but smaller cells extend back from 

 the larger ones, between the deeply staining cells of the mesoderm 

 band. 



Whilst these cells have been assuming tlie above proportions and 

 situation, the endo-mesoderm, or, as their change now shows, more 

 correctly, the mesoderm-cells of the anterior end have become 

 modified into muscle-strands (Figs. 45 and 46). The brain has 

 appeared as a darkly staining hyaline area, composed, it would 

 seem, of four or five large cells. 



