The pronephros of Chryserays marginata. 3 



genetic development from the pronephric, and are formed at a later 

 period in ontogeny. Felix believes that pro- and meso-nephric 

 tubules frequently occur in the same body-segment, and may even 

 open by the same opening into the coelom. It is unnecessary for 

 me to reopen a discussion which has been exhaustively dealt with 

 by RÜCKERT, Field, Wheeler, Felix and more recently by H. Rabl. 



Material and observations. 



The embryos of Chrysemys were procured from Mr. Albert Allen, 

 the Mendota Embryological Supply Station, Madison, Wise, U. S. A. 

 I am indebted to the Royal Society of London for a government 

 grant by which I was enabled to obtain the requisite embryonic 

 material. 



The younger embryos — ranging from the stage of segmentation 

 onwards to the stage when all the protovertebrae had been formed 

 — were fixed in Teleyesniczkt's Bichromate-acetic mixture, and 

 the older embryos with Zenker's fluid. 



Grenacher's Boraxcarmine served as a good stain for the 

 younger embryos, and Ehrlich's Haematoxylin was used for the 

 older stages. 



The embryos were stained in bulk, and sections (usually 10 — 12 fz 

 in thickness) were cut with a Cambridge Rocking microtome. The 

 method used for constructing models of the front end of the kidney 

 is described in my earlier paper. 



In order to compare in detail the development of the kidney 

 and its duct in the earlier embryos I found it very helpful to plot 

 out my results on squared paper — the side of the square always 

 corresponding to a definite number of the serial sections: in this 

 way a slight obliquity of sectioning of any embryo is eliminated, 

 and moreover such diagrams obviate a great deal of explanation. 



It is of the first importance to be able to detect any indications 

 of forward or backward shifting of the kidney relatively to the 

 body-segments, if one desires to identify with certainty any particular 

 tubule in its various embryonic conditions: hence I have given in 

 tabular form, for each of the earlier embryos examined, the extent 

 of the anterior protovertebrae on each side of the body. 



In the embryos in which the body-flexure has become strongly 

 marked it is difficult to obtain perfectly transverse vertical sections, 

 and so it becomes necessary to. establish the positions of the kidney 



1* 



