As the ovum attains maturity, important changes take place in the constitution of the germinal vesicle, which are described in the next chapter.
The egg membranes. A certain number of ova when ready to be fertilized are naked cells devoid of any form of protecting covering, but as a rule the ovum is invested by some form of membrane. Such coverings present great variety in their character and origin, and may be conveniently (Ludwig, No. [4]) divided into two great groups, viz. (1) 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.
Fig. 6. Ovum of Toxopneustes variegatus with the pseudopodia-like processes of the protoplasm penetrating the zona radiata (zr). (After Selenka.)
The primary egg membranes may again be divided into two groups (Ed. van Beneden, No. [1]), viz., (1) those formed by the protoplasm of the ovum, to which the name vitelline 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 development 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, Echinodermata, 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 protoplasm of the ovum, as has been very beautifully shown in the case of Toxopneustes by Selenka ([fig. 6]), or they admit (?) processes 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 membrane. The term zona radiata will be applied to perforated membranes of this kind. Two vitelline membranes, one perforated and the other homogeneous, may coexist at the same time, e.g. Sipunculida, Vertebrata. ([Fig. 7].)
