58 WILLIAM MOETON WHEELER, 



Äcipenser, 6—7 in Lacerta and Selachians, ca. 10 in Testudinata, 

 12 — 13 in IchiliyopMs). On any other hypothesis than that which 

 assumes the possible conversion of tubules into duct, it would be 

 necessary to suppose that reduction in the number of pronephric 

 tubules must always take place at the anterior end and proceed in a 

 cranio-caudal direction. That the reduction does take place occas- 

 ionally in this direction is shown by Petromyzon and the chick, but 

 in many forms the reduction takes place from behind forward like 

 the reduction of the gill-clefts in Craniotes. A good example of such 

 a reduction of pronephric tubules is furnished by Felix' description 

 of the Salmonid (1897 b). 



b) The Origin of the Mesonephric Tubules. 



Comparison of the renal organs of the lower Craniota with one- 

 another shows that the pro- and mesonephros by no means keep pace 

 with each other either in ontogenetic or in phylogenetic ditferentiation. 

 In other words, the pronephros of a certain form may be highly 

 specialized while its mesonephros is in a very primitive condition as 

 compared with the mesonephros of some other animal. Thus, if we 

 consider only the Myxinoid, Petromyzon and the shark, we find that 

 in the first the pronephros shows signs of high specialization in the 

 numerous uephrostomata with which each tubule opens into the peri- 

 cardial cavity. This is a condition very similar to that seen in Am- 

 phioxus, in which case also the multiple openings probably represent 

 a phylogenetic and ontogenetic departure from the primitive condition 

 of the nephridium with but a single nephrostome. In strong contrast 

 with this elaborate pronephros we have in Myxine the simplest meso- 

 nephros that occurs among Vertebrates, a metameric series of short, 

 straight tubules opening into the pronephric duct and each provided 

 with its own glomerulus. In PeLromyzon the condition is reversed: 

 the pronephros has a very simple and primitive structure, being, per- 

 haps, the most primitive and typical among Craniota, whereas the 

 mesonephros is very elaborate compared with that of the Myxinoid. 

 The tubules are not only long and convoluted but far more numerous 

 than the metameres, and the compounding of the glomeruli exhibits 

 a high degree of specialization. Finally, in the shark, the pronephros 

 is poorly developed, lacking even a recognizable glomus, and soon 

 atrophies, while the mesonephros is the "beau idéal" of a mesonephros, 

 possessing all the distinctive characters of this organ in their purity 



