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 Auodonta, 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 Chsetopods may also present 

 cases of a recun-ent cavity. 



So far, then, as the literature and my own observations go, it seems 

 that this recui-rent cavity is confined to eggs developing in fresh water 

 or moist situations, and reaches its maximum manifestation in forms 

 enclosed in thick encapsuled albumen, like Limax. In these outward 

 conditions of environment probably lies the explanation of this phe- 

 nomenon. It is probably correlated 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- 

 cesses taking place in the protoplasm of the blastomeres may be attended 

 by the endosmosis of water from the surrounding albumen and 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 in these forms that the cavity is never recurrent and rarely 

 present during cleavage. The eggs 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 



