FORMATION OF GERM-LAYERS IN CRANGON VULGARIS. 9349 
crowded than over the remainder of the egg. The distribution 
of nuclei at this stage is shown in fig. 5a. 
Shortly afterthe establishment of two ventral bands of crowded 
nuclei, each band becomes divided into three distinct regions: an 
anterior optic lobe, corresponding to the cephalic lobe described 
by Reichenbach in Astacus ;! a median region, in which the 
nuclei are less densely crowded; and a posterior thickened and 
densely nucleated region, corresponding to the thoracico-abdo- 
minal plate of Reichenbach, or to the ventral (neuro-muscular) 
plates described by Kleinenberg in the larva of Lopadorhynchus.? 
The optic lobes (fig. 6, 0. p.) are characterised by the more or 
less definitely concentric arrangement of their closely-packed 
nuclei. The region between these and the ventral plates will 
ultimately give rise to the two pairs of antenne and to the 
mandibles; while the post-mandibular ectoderm, together with 
the whole mesoderm of the body, arises from the ventral plates. 
These ventral neuro-muscular plates (fig. 6, n. m. p.) are more 
conspicuous than the remaining portions of the ventral bands, 
because their surface is externally concave (compare the section, 
fig. 16). The nuclei in these plates are arranged in irregularly 
concentric rings, like those of the optic lobes. The irregularity 
of these nuclei is perhaps exaggerated in the figure. The 
blastopore is not closed, as Kingsley erroneously supposes, but 
may be distinctly recognised as a very small pit, surrounded by 
a ring of nuclei, between the posterior margins of the ventral 
plates (see fig. 6, Blp.). This embryo, therefore, corresponds 
fairly well with Reichenbach’s stage, as shown in his fig. 3, 
pl. 11; the only important difference between the two arising 
from the very small size of the blastoporic patch in Crangon 
as compared with that of Astacus. 
There is a considerable gap between the stage just described 
and that shown in fig. 7; the changes which occur in the 
interval will, however, be readily understood from the figures 
themselves, and from the sections of intermediate stages to be 
presently described. 
1 «Studien z, Entw. d. Flusskrebsse,’ Frankfurt a/M. 
2 ¢Zeitschr. f. w. Zoologie,’ Bd. xliv, 1886. 
