356 W. F. R. WELDON. 
The anterior scattered endoderm-cells have increased in 
number since the last stage, partly, no doubt, by division of 
previously scattered cells, but partly by the migration of cells 
from the posterior endoderm. Such a migration of cells is 
distinctly indicated in fig. 22. A fair idea of the number and 
distribution of these scattered cells in the anterior part of the 
body may be gathered from figs. 18 and 19. 
The ectoderm has undergone few changes of importance. 
The optic lobes (fig. 17) have the same structure as before; 
behind these, in the region of the first antenne, the ventral 
ectoderm is slightly thickened (fig. 18), while still more poste- 
riorly the stomodzum is already visible as a slight pit, hollowed 
out in a thickened mass of ectoderm. It will be understood 
that the apparent symmetry of the mouth in fig. 19 is due to 
the obliquity of the section. 
In the region of the trunk the ventral plates are well seen 
(figs. 20 and 21) as concave thickened plates. Anteriorly 
these plates are still one eell thick, but behind the cells are in 
places arranged in two layers. In some sections (one of which I 
have purposely drawn in fig. 21) the appearances are consistent 
with the possibility that the cells of the inner layer (the future 
mesoderm) are added to the ventral plate by migration of cells 
from the primitive endoderm; but I feel convinced, from the 
examination of many series of sections, that this is not the 
case. The elongated cell on the right-hand side of fig. 20, the 
spindle in an undoubted ectoderm-cell of fig. 27, and the 
general appearance of the ventral plates in fig. 29 give evi- 
dence, of a kind which might have been indefinitely multiplied, 
that the doubling of the layers in the ventral plates is really 
due toa division of the ectoderm-cells. The doubtful case 
shown in fig. 21 has been given principally in order to do the 
fullest possible justice to Professor Kingsley’s contention that 
the primitive mesoderm is entirely invaginated from the 
blastopore. 
When the stage represented in fig. 7 is fully attained, both 
ectoderm and endoderm have made important progress. The 
endoderm, in the region of the ventral plates, is in much the 
