Embryology. 12a 
the zona pellucida, which contains—first, the proto- 
plasmic cell-substance or “ yolk,’ within which lies, 
second, the nucleus or germinal vesicle, within which 
again lies, third, the nucleolus or germinal spot. This 
description is true of the egg-cells of all animals, 
if we add that in the case of the lowest animals—such 
as sponges, &c.—there is no enveloping membrane: 
the egg-cell is here a naked cell, and its constituent 
protoplasm, being thus unconfined, is free to perform 
protoplasmic movements. which it does after the 
Fic. 30.—Ovarian orum of a Mammal, (a) magnified and viewed under 
pressure, (2) burst by increased pressure, with yolk and nucleus 
escaping: (c) the nucleus more freed from yolk-substance. (From 
Quain’s Anatomy, after Allen Thomson.) 
manner, and with all the activity, of an amceba. 
But even with respect to this matter of an enveloping 
membrane, there is no essential difference between 
an ovum of the lowest and-an ovum of the highest 
animals. For in their carly stages of development 
within the ovary the ova of the highest animals 
are likewise in the condition of naked cells, exhibiting 
amcebiform movements; the enveloping membrane 
of an ovum being the product of a later development. 
