THE DEVELOPMENT OF THE FROG 277 



excretory organs of the tadpole consist of a pronephros with 

 its duct, the segmental duct, on either side of the body, and 

 a compact mesonephros whose numerous tubules open into the 

 lower ends of the segmental duct. Towards the time of the 

 metamorphosis the pronephros is aborted, and both it and the 

 anterior part of the segmental duct eventually disappear. The 

 posterior end of the segmental duct persists in both sexes as 

 the mesonephric or Wolffian duct, which in the female func- 

 tions as the ureter, but in the male serves also as the genital 

 duct, the testes being connected with the kidneys by the 

 formation of the vasa efiferentia. 



In the dogfish the segmental duct has been described as 

 splitting longitudinally to form the Wolffian and Miillerian 

 ducts on either side, the latter persisting to form the oviducts 

 of the female, but disappearing, except for a small anterior 

 rudiment, in the male. In the frog the oviducts are not 

 formed till the end of the metamorphosis, when all four limbs 

 are well developed and the tail is being absorbed. They are 

 not split off from the Wolffian ducts, but arise independently 

 as differentiated tracts of the peritoneal epithelium, which are 

 converted first into grooves and eventually into canals. The 

 anterior end of each oviduct, which becomes the oviducal 

 funnel of the adult female, is formed as a groove of peritoneal 

 epithelium on the ventral side of the degenerating pronephros. 

 The groove is lined by a well-marked columnar epithelium, 

 which is a modification of the elsewhere flattened peritoneal 

 epithelium. In a frog which has nearly lost its tail, the groove 

 may be traced backwards for some little distance, and then 

 it becomes a canal ending in a solid thickening of the peri- 

 toneum. This thickening is continued backwards towards the 

 cloaca as a strip of columnar epithelium lying on the outer 

 border of the kidney. The strip is gradually converted into a 

 thickening, a lumen is formed in it, continuous with the lumen 

 of the short canal of the anterior end, and the whole structure 

 becomes a tube, the hinder end of which acquires an opening 

 into the cloaca at a much later date. The groove at the 

 anterior end of the tube persists as the oviducal funnel, after 

 undergoing a considerable change of position due to the 

 growth of adjacent organs. The oviduct is formed as above, 

 both in male and female frogs, but it never reaches a higher de- 

 gree of development in the male. It persists, however, and may be 



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