1889.] Embryology. 737 
there is a large yolk mass represented in all of his figures of the latter 
stages. 
In other respects the development of ^. depressa and A. polita are 
very similar. The mid-gut has its walls greatly thickened in both 
ca.ses. This is due to the hypertrophy of its constituent endodermal 
cells, which are evidentally occupied, as shown in sections of the latter 
stages, with the work of appropriating the yolk which still dilates the 
intestine. The yolk is absolutely without nuclei of its own, and 
is brownish-yellow in color. The dilated portion of the intestine 
bulges the body-wall outward into a hump-like prominence between 
the edge of the young shell and the back of the head of the embryo. 
There is a simple ctenidium or gill developed in the pallial chamber, 
and the young paired "hepatic" diverticula are greenish in color. 
The foot bears an operculum on its dorsal side long before the young 
animal leaves the egg-shell. No teeth seem to be differentiated on 
the radula in the older larva, one and a half millimeters in diameter. 
The foot, and a slight fold above the mesopodium and between the 
latter and the head, are ciliated, as well as the thickened epidermis 
about the mouth and the tips of the tentacles. The muscular mass 
underlying the radula, the otocysts and eyes, are well developed. 
The latter show pigment at this time. The yolk in the intestine has 
in my oldest stages grown quite small in amount, and causes only a 
slight projection of the body wall behind and above the head. The 
whole embryo rotates within the reddish albumen in which it is em- 
bedded, on account of the action of the cilia covering the foot. From 
the dimensions of two-thirds millimeter the embryo grows until it fills 
out the whole of the space of five millimeters in diameter inclosed by 
the egg-shell. In the course of this process the albumen surrounding 
the embryo is probably swallowed by the young snail and appropriated 
by the hypertrophied amoeboid cells of the intestinal wall. 
The air-vesicle is always at the upper pole of the egg, and forms a 
lenticular cavity just within the calcareous shell. It doubtless has to 
do with the respiration of the embryo, and recalls in a striking way 
the vesicula aeris of the bird's ovum, except that it has a different posi- 
tion, and does not seem to be separated from the albumen by a mem- 
brane.— John A. Ryder. 
Development 01 C^a^gon vulgaris. »— Dr. Kingsley's third 
paper on the development of this crustacean has just been published. 
His general conclusions are interesting as pointing out the presence of 
