CHAPTER Xril. 



THE FEMALE llEPKODT (TIVE SYSTEM. 



A:mong the vertebrates, the sexual systems are generally separated, 

 difi'erent infli\iduals developing into either male or female, but for 

 a time during the early embryology the male and female sexual 

 systems follow the same developmental plan and are indistinguish- 

 able either anatomically or histologically. P^lements of both sys- 

 tems are developed at this time, though genetically sex is determined 

 at the time of fertilization. In early vertebrate embryos the uro- 

 genital fold develops on either side of the dorsal mid-line. From 

 the greater part of this fold the urinary system develops, as already 

 described; but from the mesial ventral surface a longitudinal thick- 

 ening arises to form the genital ridge. This ridge is covered with 

 a cuboidal epithelium and separated from the developing kidney by 

 mesenchyme. The simple epithelium of the genital ridge prolifer- 

 ates into a stratified zone of polyhedral cells in which two types of 

 cells soon make their appearance. Larger spherical cells, the pri- 

 mordial sex cells, are scattered among the cuboidal or polyhedral 

 indifferent cells, and the underlying mesenchyme forms string-like 

 masses projecting into the epithelial mass. This mass represents 

 the early indifferent stage in development of the sex glands, or 

 gonads. The excretory ducts of the genital system' are also closely 

 associated with the urinary system. Two longitudinal ducts are 

 developed on each side from the mesoderm lining the ccielome, and 

 these open caudally into the cloaca. The one duct is the Wolffian 

 duct, already described in considering the excretory system, the 

 other is the ]Mullerian duct, which becomes the oviduct of the female. 

 With development, sexual differentiation occurs, and the system 

 associated with the sex so determined progressively differentiates 

 while the elements of the other system degenerate. 



THE OVARIES. 



The female organs of reproduction consist of the ovaries, in which 



the ova are produced, and a pair of ducts, the o\iducts, by which 



the ova are conducterl away. The ovaries of vertebrates are usiudly 



paired organs, but occasionally, as in elasmobranchs and birds, one 



14 



