GENERAL HISTORY. 



FIG. 6. OVUM OF Toxo- 



PNEUSTES VARIEGATUS WITH 

 THE PSEUDOPODIA-LIKE PRO- 

 CESSES OF THE PROTOPLASM 

 PENETRATING THE ZONA RADI- 



ATA (zr). (After Selenka.) 



their character and origin, and may be conveniently (Ludwig, 

 No. 4) divided into two great groups, viz. (i) those derived from 

 the protoplasm of the ovum itself or from its follicle, which may 

 be called primary egg-membranes; and (2) those formed by 

 the wall of the oviduct or otherwise, such as the egg-shell of a 

 bird, which may be called secondary 

 egg-membranes. 



The primary egg-membranes may 

 again be divided into two groups 

 (Ed. van Beneden, No. 1), viz., (i) 

 those formed by the protoplasm of 

 the ovum, to which the name vitel- 

 line membranes will be applied ; 

 and (2) those formed by the cells of 

 the follicle, to which the name 

 chorion will be applied. 



The secondary egg-membranes 

 will be dealt with in connection with 

 the systematic account of the develop- 

 ment of the various groups. They 

 coexist as a rule with primary membranes, though in some 

 types (Cephalophorous Mollusca, many Platyelminthes, etc.), 

 they constitute the only protecting coverings of the ovum. 



The vitelline membranes are either simple structureless 

 membranes or present numerous radial pores. Membranes 

 with the latter structure are very widely distributed, Echino- 

 dermata, Gephyrea, Vertebrata, etc. ( Vide figs. 5 and 7.) The 

 function of the pores appears to be a nutritive one. They either 

 serve for the emission of pseudopodia-like processes of the pro- 

 toplasm of the ovum, as has been very beautifully shewn in the 

 case of Toxopneustes by Selenka (fig. 6), or they admit (?) pro- 

 cesses of the follicular epithelial cells (Vertebrata), Their 

 presence is in fact probably caused by the existence of such 

 processes, which prevent the continuous deposition of the mem- 

 brane. The term zona radiata will be applied to perforated 

 membranes of this kind. Two vitelline membranes, one per- 

 forated and the other homogeneous, may coexist at the same 

 time, e.g. Sipunculida, Vertebrata. (Fig. 7.) 



The chorion is often ornamented with various processes, etc. 



