140 Elementary Zoology. 



cceloni by three ciliated funnels, or nephrostomes. The exact 

 way in which these are formed appears to be this : the rod is, 

 as said, at first solid ; it then becomes grooved along the inner 

 surface, and ultimately the groove closes and shuts off a cavity, 

 lying therefore within the rod — except at three points, which 

 are the nephrostomes. The portions of the tube bearing the 

 nephrostomes grow out into short tubes ; these tubes become 

 later branched and complicated. This structure is called the 

 head kidney, and in the tadpole it plays the part of an excretory 

 organ ; it is later replaced by the mesonephros, and becomes 

 degenerate, ultimately disappearing. The head kidney, or 

 pronephros, as it is better to term it in correspondence with 

 the mesonephros, is interesting as a relic in the frog, for in 

 the marsipobranchs, or cyclostomata, it persists throughout life 

 as a functional organ of excretion. 



In the chick the pronephros is not only never developed 

 as a functional organ of excretion, but it actually appears later 

 than the mesonephros. As in the tadpole, the first part of 

 the excretory system to be developed is the longitudinal duct, 

 which corresponds to that of the tadpole. It acquires its 

 lumen, however, by canalization, not by the closing of the 

 lips of a groove — a difference, perhaps, of not great im- 

 portance. It may be termed the archinephric, or segmental 

 duct. Into this duct (there are, of course, two ducts, one on 

 each side of the body) open a number of short tubules, the 

 tubules of the mesonephros or Wolffian body. The anterior set 

 of these tubules develops in two sections ; a depression of the 

 coelom is formed, and within the mesoblast a coiled tube ; the 

 depression or nephrostome becomes continuous with the tube, 

 and the latter with the longitudinal duct. This anterior set of 

 tubules presently degenerates and disappears. The posterior 

 set of tubules differs in having no nephrostomes. 



Later, the rudiments of the pronephros put in an appear- 

 ance. They are three depressions of the lining membrane of 

 the ccelum (the number, it will be noted, corresponds to what 

 is found in the frog), which become connected by a ridge of 

 tissue. The anterior of the three funnels persists, and forms 

 the mouth of the oviduct in the female; the ridge of tissue 



