358 -W. F.-R. WELDON. 
entirely free from the body in the section figured. Thesecond 
antennz are represented simply by densely nucleated portions 
of ectoderm (fig. 25) which do not as yet project beyond the 
surface, while the mandibles are already visible as slight pro- 
jections (fig. 26, Md.). 
The ventral plates (fig. 27, ». m. p.) are thicker than in pre- 
vious stages, both because the individual cells are larger, and 
because they are arranged more completely than before in a 
double layer. The section figured is especially fortunate, 
because the spindle which appears on the right-hand side shows 
distinct indication of a transverse division of the ectoderm-cell 
to which it belongs. The very remarkable appearance of the , 
nuclei of the left ventral plate may, perhaps, be an indication 
that these also are about to divide. 
In the next stage (fig. 8) the principal changes affect the 
external form. The endoderm is practically unaltered, except 
for the further addition of wandering anterior cells to the ventra] 
cephalic layer. The ectoderm is pulled out, as already stated, 
into well-marked processes which form the three pairs of 
appendages, and the structure of the optic lobes is slightly 
more complex than before. The division into a retinal plate 
(fig. 28, r. p.) and a ganglionic region (0. g.) is sufficiently 
obvious. The ganglia are confluent in the middle line, and partly 
overlap the retinal plate. The section figured being somewhat 
oblique, the retinal plate appears free on one side, while on the 
other it is partly covered by nervous cells. 
The ventral plates (fig. 29, n. m. p.) have much the same 
appearance as that seen in older embryos of the last stage ; they 
are, however, still more distinctly two-layered. Between these 
plates, in the middle line, the appearance of a fusion between 
ectoderm and endoderm is still retained. 
With the growth of the thoracico-abdominal papilla the 
endoderm and the ventral plates undergo important changes. 
A horizontal longitudinal section through the papilla of an 
embryo intermediate between figs. 7 and 8 is shown in fig. 30. 
With the growth of the thoracico-abdominal papilla, the poste- 
rior endoderm is drawn upwards into the cavity of that 
