506 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



gradually breaking up, and the female becoming sur- 

 rounded with follicular cells and growing at their 

 expense. The epithelial cells that are about to be- 

 come germinal are, in the early stages of both sexes, 

 formed on a genital ridge, or " sexual eminence " ; 

 but, while this ridge becomes more prominent in the 

 female, it gradually disappears in the male. In. 

 other words, the distinction between the two sexes 

 is, at this point, well marked at a very early period. 



With the exception of Ampliioxus, no Chordate 

 presents any distinct indications of the metameric 

 segmentation of its reproductive organs, and the 

 consolidation which was observed in so many of 

 the non-vertebrated forms is just as well marked 

 in this phylum. The higher the vertebrate, the 

 more compact its genital glands. 



In Petromyzoii the testis extends almost through- 

 out the whole length of the body cavity ; in the 

 Elasmobranchs it is best developed in the anterior 

 region of the body ; and in most Ganoids it is 

 rather smaller than in Elasmobranchs ; in the 

 Teleostei the glands are known to vary much in 

 size at different times of the year, but they are 

 generally of considerable length. In the Ca^cilire 

 the testes are broken up into a number (about ten) 

 of separate parts, connected one with another by 

 ducts, and having, on either side, the appearance of a 

 string of pearls. In the Urodeles and Anura the 

 testes are more or less elongated or rounded, but 

 never form, externally, more than a single mass. As 

 we pass through Reptiles to Birds we observe the same 

 phenomenon, though it is necessary to note that the 

 size of the organs varies very much with the conditions 

 of virile activity ; for example, the testes of the 

 sparrow, which in January are only two millimetres 

 long, are in April 15 mm. long, and wider in pro- 

 portion. 



