408 s. HATTA : 



ment, the physiological function, and the relation of the vascular 

 system to the organ. His comparison is, however, almost entirely 

 limited to Selachia on the side of Craniota, owing perhaps to the 

 scantiness of the literature at that time. I accept in the main 

 this homology, and I may perhaps extend this comparison a little 

 further. 



I will begin with the homology of the pronephros of Cyclo- 

 stomata with the " Nierencanälchen " of Amphioxm. 



It is well known that the starting point of the hepatic 

 diverticulum from the enteric canal demarcates, in the Chordata, 

 the respiratory section of the canal from tlie nutritious section of 

 it ; and, as Gegenraue ('78, pp. 563 — 581), Balfoue^^ {'^b), and 

 others affirm, the œsophagus and stomach in the higher forms are a 

 part of the former section, which is called the fore-gut. And the 

 homology of the hepatic cœcum of Amphioxus with the liver of 

 the Craniota, lias been much strengthened by recent mor- 

 phological studies and physiological experiments-^ The results 

 of my present study also confirm this view. I will use, therefore, 

 this fixed point as the landmark of comparison of the two 

 organ- systems, the pronephros and the " Xierencanälchen," and of 

 the pronephros in different groups of Craniota. 



l)Froin the account of Balfouk, I will cite the following lines:— 



' Jn Amphioxus tlie respiratory region extends close np to the opening of the hepatic 

 diverticulum, and therefore to a position corresponding with the conimencenient of tlie 

 intestine in higher types. In the craniate Vertebrata the number of the visceral clelts has 

 become reduced, but from the extension of the visceral clefts in Amphioxus, combined with 

 the fact that in the higher Vertebrata the vagus nerve, which is essentially the nerve of the 

 branchial pouches, supplies, in addition tlie walls of the oesophagus and stomach, it may 

 reasonably be concluded, as has been pointed out by Gegenbaur, that the true respiratory region 

 primitively included the region which in the higher types forms tlie oesophagus and 

 stomach' (Vol. ii, p. 7nS). 



Balfour has also shown that the solid cord of the œsophagus in Elasmobrancliii and 

 Teleostei, is the remanent of the gill-rudiments in tlie ancestry {loco cit., pp. CI and 78). 



2) J. A. Hammar, '99, '98, and Guido Schneider, '99, 



