104 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 it has been 

 suggested that in others two or more primitive ova are con- 

 cerned 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 tn th ern the greate r si/e a.nd npsriity.nf 

 the permanen t OYum~~ai'e~ cIiiefly 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 

 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. 

 , Aiittle later ■Still-arlaj£er_x)£— bla^- pigment appflar s on the 

 _ginjaffi of thft-ov»m^-it--is-a,t-£iatJLixsgalajJy.diatdfcu te ov er 

 the whole surface, but as the ovum ripens it becomesrestr icted 



J;o-one-ialf-piL_hemispherei_ The .pigment is contained, and 



-afftt rou i ly form ed' within-4he- ovum itself, but'TF"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 



