FORMATION OF GERM-LAYERS IN CRANGON VULGARIS. 357 
same condition as before, but the lumen, which was at least 
occasionally present during the earlier stages, has now entirely 
disappeared ; and the whole posterior portion of the endoderm 
is now a branched vacuolated mass. The blastoporic area is 
no longer conspicuous in surface views of the embryo, though 
its position is indicated by the angle between the posterior 
portions of the ventral plates (fig. 7, Blp.) ; but the region 
of continuity between ectoderm and endoderm in the middle 
ventral line is even more conspicuous than before (fig. 27). 
The separation between the lateral endoderm and the ventral 
plates is even more conspicuous than before. The cells of the 
anterior endoderm have become more abundant; and while 
certain of their number remain scattered through the yolk, 
others are beginning to arrange themselves in a layer on the 
ventral surface of the yolk, so that the alimentary canal begins 
to acquire a definite epithelial floor. This layer of endoderm 
is already fairly complete in the region of the mandibles and 
of the second antennez, while further forwards it is still im- 
perfect (compare figs. 283—26, En’). Posteriorly, the epithe- 
lial layer of anterior endoderm becomes continuous with the 
posterior mass already described. This anterior layer of endo- 
dermal epithelium appears to correspond, on the one hand, 
with the similar layer of anterior endoderm described by 
Nusbaum in Mysis, and on the other with the layer of 
flattened cells, fusiform in section, which are stated by 
Kingsley to represent the cephalic mesoderm. 
The ectoderm has become specialised, in the region of each 
optic lobe, into a retinal plate, which is one cell thick, and a 
ganglionic region, consisting of several layers of cells. The 
appearance of the optic lobes in section is nearly the same as 
that shown in fig. 28, from an older embryo. The first 
antenne appear immediately behind the optic lobes (fig. 28, 
Ant. i), and at the inner side of the base of each is an aggre- 
gation of ectodermal nuclei representing a commencing ganglion 
(fig. 23, N. S.). Five sections behind the level of fig. 23 
appears the stomodzum (fig. 24, Stom.), on each side of which 
is an aggregation of nervous nuclei. The antennules are almost 
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