262 Miscellaneous. 
MISCELLANEOUS. 
On the Earliest Stages in the Development of Sessile-eyed Crustacea. 
By M. Louis Rove. 
I nave had the honour of communicating to the Academy several 
of the most important phenomena presented by the sessile-eyed 
Crustacea in the course of their embryogeny; some more recent 
investigations enable me to complete the knowledge already acquired 
and to prepare a synthesis of the first stages of the development, 
taking as types Asellus aquaticus and Porcellio scaber. 
The ovule is always rich in nutritive vitellus ; nevertheless the 
bulk occupied by the latter varies according to the species. When 
it is least in amount the fertilized ovum undergoes a total and radial 
segmentation, the segments assuming the well-known form of cones, 
of which the apex is turned towards the centre of the ovule and the 
base towards the periphery; on the contrary, when its quantity is 
considerable, as in Porcellio for instance, this preliminary segmen- 
tation is not manifested. But, whatever be the mode exhibited, 
after the radial division when it exists, or from the moment that the 
ovum is mature when it does not-appear, the formative vitellus 
(‘“ vitellus évolutif”) does not remain mingled with the nutritive 
vitellus, but separates from it. ‘This separation does not manifest 
itself at the same time throughout the ovum; it commences in a 
zone which corresponds to the future anterior extremity of the 
embryo. The formative vitellus forms in the first place in this 
region a little cicatricle, which rapidly organizes itself into cells, to 
which the nuclei are furnished by the conjugated nucleus, which 
results from the fusion of the male and female pronucleus effected 
in fertilization. Fresh quantities of formative vitellus then become 
isolated from the nutritive vitellus and added to the cicatricle, in- 
creasing its mass and dividing likewise into cells; in this way the 
cicatricle grows and gradually envelops the nutritive vitellus, advan- 
cing with regularity from the zone which it occupied until it reaches 
the pole diametrically opposite; a cellular layer is extended by this 
proceeding upon the periphery of the ovule, and finally surrounds it. 
Arrived at this stage of development, the embryo is constituted 
by a layer of cells which surrounds a compact mass of nutritive 
vitellus ; this layer is the blastoderm, which will give rise to the 
three blastodermic layers. ‘To this end the cells of the blastoderm 
produce a large number of cellular elements, of which some pene- 
trate into the nutritive vitellus, while the rest intercalate themselves 
between the latter and the blastodermic layer; the development 
of the two kinds of cells is similar. Several of these elements, col- 
lected in two groups placed at the sides of the embryo and not far 
from the medio-ventral line, arrange themselves in two symmetrical 
layers which penetrate into the nutritive vitellus, converging towards 
one another ; these two layers, separated from the time of their first 
appearance, represent the rudiments of the endoderm. The other 
elements do not arise in limited zones; they are produced by the 
blastoderm throughout its entire extent, and give rise to the meso- 
