FORMATION OF THE EGG 105 



The Primitive Ova. At an early stage certain of the 

 epithelial cells of the genital ridge become conspicuous by 

 their larger size and more spherical shape ; and around these 

 larger cells, or primitive ova, as they are called, the smaller 

 epithelial cells become arranged so as to form capsules or 

 follicles ; the follicles, with their contained primitive ova, 

 form small knob-like projections on the surface of the 

 genital ridge. New primitive ova arise either directly from 

 the surface epithelium, or by division of the already existing 

 ones. 



The Permanent Ova. Up to this time there has been no 

 distinction between male and female, the processes described 

 occurring in all tadpoles alike. Sexual differentiation appears 

 about the time of the metamorphosis. In the female the 

 change consists essentially in a great increase in the size of 

 the genital ridge, which now becomes the ovary, and in the 

 formation of the permanent ova, or eggs. The permanent 

 ova are derived from the primitive ova ; in some cases each 

 primitive ovum is directly converted into a permanent ovum, 

 but it has been suggested that in others two or more primitive 

 ova are concerned in the formation of a single permanent 

 ovum. 



A permanent ovum is enclosed in a follicle or capsule like 

 the primitive ovum, and differs from this latter in the following 

 points : (1) it is of larger size ; (2) it contains within its sub- 

 stance a number of small sharply defined yellowish granules of 

 food yolk, which are elaborated by the follicle cells and passed 

 on from them to the ovum ; these yolk granules increase 

 rapidly in number, and to them the greater size and opacity of 

 the permanent ovum are chiefly due ; (3) important changes 

 have occurred in the nucleus; in the primitive ovum the 

 nucleus is small, granular in appearance, and apparently solid ; 

 in the permanent ovum the nucleus, or germinal vesicle, is of 

 very large size, up to half the diameter of the entire ovum, 

 and consists of an elastic capsule or nuclear membrane, 

 filled with fluid and traversed by a protoplasmic reticulum 

 enlarged at its nodes to form the nucleoli, or germinal 

 spots. 



When the permanent ovum has reached a diameter of about 

 0'5 mm., an exceedingly thin structureless investment, the 



