FORMATION OF GERM-LAYERS IN CRANGON VULGARIS, 8355 
bryonic area; but in the region of the ventral bands the 
thickening is greater, especially in the optic lobes and in the 
ventral plates. The optic lobes (fig. 16, 0. p.) consist of long, 
closely-packed cells, with elongated nuclei, the cells of the 
ventral plate being broader in proportion to their length, with 
more nearly spherical nuclei. The concavity of the ventral 
plate is fairly well seen in fig. 16. 
The condition of an embryo intermediate between figs. 6 
_ and 7 is represented in figs. 17—22. The endoderm is seen to 
differ from that of younger embryos chiefly in the greater size 
of the posterior portion, which is still continuous with the 
blastoporic area. In the specimen from which the transverse 
sections, figs. 20 and 21, were prepared I could find no trace of 
a lumen in the posterior endoderm. An egg, taken from the 
same mother, was cut longitudinally, and the median section 
(fig. 22) shows a distinct cavity, which does not communicate 
with the exterior. The lumen is present in about half my 
series of sections through embryos of this age. It is very 
difficult to determine whether the increase in the amount of 
posterior endoderm which has taken place since the last stage 
is due to a continuation of the process of invagination, or to 
simple division of the previously invaginated cells. The 
appearances indicated by figs. 20—22 seem equally consistent 
with either view. An interesting feature of the posterior endo- 
derm is its apparent tendency to become continuous with the 
ectoderm in the middle ventral line. The section drawn in 
fig. 21is near the anterior limit of the original blastoporic area, 
and that shown in fig. 20 is the sixth of a fairly thick series of 
sections in front of it. In the anterior section the large endo- 
derm-cell seemed to be distinctly continuous with the small 
cells between the ventral plates. This kind of appearance will 
be noticed in sections through the later stages. It is, of 
course, possible that these appearances of a fusion between 
endoderm and ectoderm are merely accidental ; but it is equally 
possible that they are indications of a ventral elongation of 
the blastopore, in which case they will be regarded by many 
morphologists as of great importance. 
