196 THE FROG. 



but there is no doubt that the greater part of the length, and 

 perhaps the whole, of the duct arises independently of the 

 Wolffian duct; and apparently from a modified strip of perito- 

 neal epithelium which runs parallel to the Wolffian duct, along 

 its outer side. 



Towards the close of the metamorphosis, and during the 

 absorption of the tail, a patch of modified peritoneal epithelium 

 appears on the ventral surface of the now rapidly disappearing 

 head kidney, below and to the outer side of the nephrostomes. 

 of which, as a rule, only one persists at this stage (Fig. 88, KS). 

 This patch of epithelium differs from the general peritoneal epi- 

 thelium in its cells being columnar instead of squamous in shape. 

 A longitudinal groove forms on the surface of this patch of 

 columnar epithelium, to the outer side of the nephrostome ; and, 

 by fusion of its lips, the groove becomes a tube, opening in front 

 into the body cavity, and ending blindly behind. This tube 

 is the first formed part of the Miillerian duct ; it lies very close 

 to the anterior end of the segmental duct, but it is not clear 

 that there is any connection between the two ; it is quite in- 

 dependent of the nephrostome, which closes up and disappears 

 very shortly after this stage. 



The Miillerian duct extends forwards, as an open groove, some 

 distance in front of the point at which the duct is first formed : 

 and, by closure of the lips of the groove from behind forwards, 

 the mouth of the duct is carried forwards, outwards, and down- 

 wards, round the anterior end of the body cavity, to its adult 

 position at the root of the lung. 



The growth backwards of the Miillerian duct is effected by 

 a longitudinal band of cells, which appears a little to the outer 

 side of the Wolffian duct, but quite independent of this, and 

 apparently derived directly from the peritoneal epithelium. This 

 band, which is continuous with the hinder end of the Miillerian 

 duct, ultimately becomes tubular, and acquires posteriorly an 

 independent opening into the cloaca. 



In the male frog the Miillerian duct stops at this stage. 

 In the female it undergoes further changes by which it becomes 

 converted into the oviduct. It increases in length, becoming 

 sinuous, and ultimately convoluted along the greater part of its 

 length ; its walls thicken greatly, and the epithelium lining them 

 gives rise to the gelatinous secretion which is poured out over the 



