104 DEVELOPMENT OF THE FROG 



hollow ball pushed in at one pole while the space between 

 the inner and outer cavities was filled up by growth of 

 tissue round their junction. Or, starting with a solid ball, 

 imagine a liquefaction to occur at the centre, an inner layer 

 of tissue condensing round this central cavity and round the 

 outer surface. We must now anticipate that the latter 

 process is the one chiefly followed by the frog, and that it 

 starts at the lower pole of the ball. Thus we come to the 

 conception of the triple-layered embryo arising from a solid 

 or nearly solid mass of cells whose upper and lower poles 

 differ : and from this it is an easy step to the next, a single 

 cell with different poles. This cell is the egg, whose fortunes 

 we shall now follow in the reverse direction. 



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 modifica- 

 tion in the shape of the peritoneal epithelial cells which, else- 

 where 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. 



