256 T. H. Burtenp, 
The mesonephric tubules have a development still more arrested 
than is that of the duct into which they open; they have a dual 
constitution, one portion (malpighian capsule) arising from the 
nephrotome, and the rest consisting of a pronephric tubule which is 
modified by the environment. 
Although there is a regular metamerism in the anterior region 
of the mesonephros, the pronephros is not metameric. 
The following considerations militate against the view that the 
pronephros is a {larval excretory organ of definite extent and com- 
parable to the larval organ of many Invertebrates. The region which 
gives rise to the functional pronephros is somewhat variable in 
position and extent in different, though closely allied species, and 
even within the same species; the pronephros is of greater extent 
in lower Vertebrates than it is in higher forms generally speaking, 
although there are noticeable cases (like that of the Dogfish) where this 
is not the case; the organ is a prominent functional structure for 
a comparatively long period in some Vertebrates (Myxine, Chrysemys etc.) 
whilst in others (Dogfish) its functional activity is of very limited 
duration apparently. 
