KOFOID: DEVELOPMENT OF LIMAX. 103 
Neritina, but the cleavage cavity does not seem to be prominent here. 
It is however well marked in Amnicola. 
Among the Lamellibranchs there is the same absence of reference to a 
cleavage cavity in marine forms, but its recurrent nature is noted in 
Unio and Anodonta, probably also in Cyclas. A cavity also occurs in 
Dreissena, but we do not know that it is recurrent. It seems probable 
that the encapsuled eggs of the non-marine Cheetopods may also present 
cases of a recurrent cavity. 
So far, then, as the literature and my own observations go, it seems 
that this recurrent cavity is confined to eggs developing in fresh water 
or moist situations, and reaches its maximum manifestation in forms 
enclosed in thick eneapsuled albumen, like Limax. In these outward 
conditions of environment probably lies the explanation of this phe- 
nomenon. It is probably eorrelated with the nutritive and excretory 
processes of the egg, especially the excretory, as Warneck long ago sug- 
gested, for we have no evidence that the embryo depends in the cleav- 
age stages upon the surrounding albumen for its nutriment, The latter 
serves mainly as a protective covering during these stages, though later 
it is certainly used as food by the growing embryo. The metabolic pro- 
cosses taking place in tho protoplasm of the blastomeres may be attended 
by the endosmosis of water from the surrounding albumen aud its sub- 
sequent exosmosis. That part which passes out from the cell along the 
facets of contact with the other cells or on its inner surface, when such 
exists, contributes toward the formation of a cleavage cavity in some 
of its varying manifestations. When however the egg, or its envelopes, 
is bathed by a changing medium, as is the case with fresh-water and 
marine forms, the cavity is reduced or is altogether wanting, it being 
probable that the changing medium facilitates the solution and removal 
of the waste products from the surface of the egg. This is especially 
true of marine forms where the presence of the salt in the bathing 
medium doubtless facilitates the solution of the albuminous matters, 
and it is im these forms that the cavity is never recurrent and rarely 
present during cleavage. The eges of the land Pulmonates, lacking as 
they do the salt, or even a changing medium, have the further disadvan- 
tage of a coating of dense albumen. — This and its thick envelopes must 
necessarily impede the processes of excretion and respiration, a hin- 
drance which may be in part removed by the increase of the osmotic 
surface attendant upon the formation of the cleavage cavity, and also by 
the forcible removal of the products of excretion by the expulsion of 
the contents of the cavity. "That there exists a physiological necessity 
