2 T. H. Buhlend, 



1. The Rückeet-Semon Theoiy, supported in the main by the work 

 of VAN WiJHE (1889), Hoffmann (1889), Wiedeesheim (1890), Rabl 

 (1896), Maas (1897), Wheelee (1899), Hatta (1900) etc. 



The chief contentions of supporters of this theory are: 



a) that pronephric tubules develop in a different way to that in 

 which mesonephric tubules develop; 



b) that in the pronephric reg^ion at a later period mesonephric 

 tubules may arise, thus producing an overlapping of the pro- 

 and meso-nephros ; 



c) that the mesonephros develops much later than the pronephros. 



For these reasons the mesonephros is regarded as quite distinct 

 from, and not homodynamous with, the pronephros. 



2. The Sedgwick-Field Theory, which receives support from 

 the investigations of many workers, especially those of Peice (1896) 

 and Beauee (1902) etc. 



This theory maintains that the pro- and meso-nephros are homo- 

 dynamous, and attributes any morphological differences between these 

 two parts of a common organ to the later development of the meso- 

 nephros, which is correspondingly modified. Tlie larval life is said 

 to influence the degree of development and duration of the pro- 

 nephros, forms having no larval life — e. g. Selachians and Amniotes 

 — having no functional pronephros. 



Peice (1896) gave an account of Bdellostoma. The manner of 

 the kidney development in this animal certainly gives support to the 

 above theory. First the primitive organ is developed — for which 

 Peice suggests the name "holonephros" — and secondly a differen- 

 tiation into anterior pro- and posterior meso-nephros occurs, or at 

 least Peice interprets his observations in this way. 



3. Felix has promulgated a view which is a modification of 

 the Rückeet-Semon theory, but since the author has devoted so 

 much time and labour to this matter, and since moreover he is entitled 

 to be regarded as one of the leading authorities on the subject of 

 the development of the vertebrate excretory organ, his opinion needs 

 special mention. 



Felix appears to regard the pro- and mesonephros as not liomo- 

 dynamous, and explains in detail characteristic distinctions between 

 the two; but he regards them as structures derived from the same 

 stem ; the mesonephric tubules are however separated in their phylo- 



