272 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY 
their deep staining and in their minute structure very conspicuously 
from the nuclei of the pronephric tubules and of the vascular endo¬ 
thelium. In all three forms above mentioned, the pronephros is 
without capillaries, but has instead sinus-like vessels, the endothelium 
of which is in large part in close contact with the epithelium of the 
pronephric tubules. 
The peculiarity of the pronephric circulation has been noted by 
previous writers. For example, the sinus-like character and the 
intimate relation to the veins were noted by Alexander Gotte' in 
1875, see his Entwickelimgsgeschichte der unke, p. 760, also by M. 
Fiirbringer, 76 . 1 , Taf. I, fig. 4. In W. Felix’s recent admirable 
monograph, 97 . 1 , the similar conditions in teleostean pronephros 
are referred to. The list of authorities upon this feature of the 
pronephros in Iclithyopsida might be considerably extended, in short 
its seems to me that one may safely regard the multifid intertubular 
venous sinus as an essential characteristic of the well-developed 
pronephros. 
The facts presented bring us to question : Is there a constant and 
therefore typical difference between the pronephros and mesonephros 
on the one hand and the true kidney on the other as regards the 
form of circulation, a blood sinus developing about the tubules of 
the former, true capillaries about the tubules of the latter ? The 
present possibilities indicate an affirmative answer, and such an 
answer would, in my opinion, go far towards establishing a morpho¬ 
logical difference between the various renal organs and would tend 
to prove that the true kidney (metanephros) is not phylogenetically 
related to the mesonephros, but is a wholly new acquisition, not 
evolved from segmental organs (nephrotomes). 
