128 Miscellaneous. 
ultimate position, extends in the direction of the longitudinal axis 
of the body. 
The important fact in this series of phenomena is the singular 
manner in which the curvature is produced. The latter does not 
take place gradually, by the folding upon itself of the young 
embryo; it appears all at once, by means of a cleft, which pene- 
trates into the cicatricle, divides it into two planes, and increases 111 
size with the latter. This cleft does not enter into the composition 
of the body ; it forms part of the surrounding space and is destined 
to disappear. Authors have already alluded to its presence; but 
in my opinion they have strangely mistaken its real value. They 
have considered the rudiment of it as corresponding to a gastrular 
invagination, and this as much on account of its very early appear- 
ance as of its origin and mode of growth. On carefully foliowing 
the series of embryonic stages, we perceive that this invagination in 
no way contributes to the formation of the digestive cavities, it 
remains independent of these, preserves its autonomy while increasing 
in size, and finally becomes the cleft which, in the embryo doubled 
upon itself, separates the cephalothorax from the abdomen. 
A division of this kind is not peculiar to certain of the higher 
Crustacea ; it exists, although less pronounced, in other more simple 
representatives ot the class—in Asellus aquaticus, Fabr., for 
example. The embryogeny of the latter proceeds, as regards the 
extension of the cicatricle, similarly to that of Porcellio, at least in 
its general features; only the body of the young emkryo, which is 
at first spherical, assumes the definitive elongated form by cleaving 
in its dorsal region, leaving the ventral portion undivided and sepa- 
rating the halves one from the other after the manner of two valves. 
This phenomenon is most important, for it represents in a relatively 
simple condition and at an advanced stage of development the pre- 
cocious fission of the cicatricle in the prawn. 
With reference to comparative embryology, this fission corresponds 
to a displacement in space. The body of the embryo enclosed in its 
egg-shell comes all at once, by its means and without any other 
modification, to acquire a recurved shape, which enables it to be 
contained in the cavity limited by the shell. Undoubtedly the cause 
in such a case must be sought for in the abundance of the food- 
yolk: the latter renders the ovum very bulky and prevents all 
folding back. ‘This displacement involves very remarkable conse- 
quences with respect to the position of the rudiments of several 
organs. ‘Thus, the extremity of the abdomen arises beneath and in 
front of the head; the anus breaks through immediately behind the 
mouth, and soon. ‘These phenomena proceed from the foregoing. 
All the organs are afterwards restored to their places by the 
straightening of the body, the effacement of the curvature, and the 
disappearance of the cleft. Among the alterations introduced into 
the embryonic processes by the presence in the ovum of a large 
quantity of food-yolk, the production on the spot of a doubling back 
of the entire animal, by means of the simple formation of the free 
space which separates the two folded portions, is one of the most 
curious.—Comptes Rendus, t. cxx. no. 5 (February 4, 1895), 
pp. 271-273. 
