FORMATION OF THE EGG 103 



recapitulate, in a moi'e or less modified or abbreviated manner, 

 the past ancestral history of the .species to which it belongs ; 

 is compelled, in fact, to climb up its own genealogical tree. 



That frogs should in their early stages be fish is, therefore, 

 to be understood as meaning that frogs are descended from 

 fish-like ancestors, and repeat this history in the course of their 

 own development. 



II. Detailed Account. 

 A. Formation of the Egg. 



The early stages in the formation of the egg cannot be seen 

 in the adult frog, but must be studied in tadpoles.. In tadpoles 

 of about 10 mm. length, shortly after the opening of the mouth, 

 a pair of longitudinal ridge-like thickenings of peritoneum 

 appear along the dorsal surface of the body cavity, close to the 

 mesentery,and along the inner borders of the developing kidneys. 

 These genital ridges are found in all tadpoles of this age, no 

 distinction of sex appearing until a much later period. 



Each genital ridge is at first due merely to slight modification 

 in the shape of the peritoneal epithelial cells which, elsewhere 

 flattened, become here cubical or slightly columnar. The ridges 

 soon become more prominent, especially at their anterior ends ; 

 their growth being due partly to the epithelial cells increasing 

 by division so as to form a layer several cells thick, and partly 

 to the ingrowth of an axial core of connective tissue from the 

 basal membrane of the peritoneum along which the bloodvessels 

 gain access to the ridge. 



From the posterior two-thirds of the genital ridge the ovary, 

 or in the male the testis, is developed ; while the anterior third 

 undergoes degenerative changes, and becomes converted into the 

 fat body. 



The Primitive Ova. At an early stage certain of the epi- 

 thelial 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, forming 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. 



