114 DEVELOPMENT OF THE FROG. 



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 in others two 

 or more primitive ova appear to be 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 recticulum 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 

 vitelline membrane, is formed immediately around it, within 

 the follicle. The mode of origin of the vitelline membrane is 

 not clearly made out, but it seems to be formed from the ovum 

 itself rather than from the follicular epithelium. 



A little later still a layer of black pigment appears on the 

 surface of the ovum ; it is at first irregularly distributed over 

 the whole surface, but as the ovum ripens it becomes restricted 

 to one half or hemisphere. The pigment is contained, and 

 apparently formed within the ovum itself, but it is not clear 

 how it is formed or what purpose it fulfils. 



B. Maturation of the Egg. 



The eggs have now reached their full size, and project from 



