MUSEUM OF COMPARATIVE ZOOLOGY. 203 



geon U. S. Army, for the favor of sending me from the Surgeon General's 

 library in Washington a number of papers to which I should otherwise 

 have been unable to gain access. I am further indebted to Mr. Samuel 

 Garman and to Mr. G. H. Parker for the revision of my proof-sheets, and 

 for suggestions during the progress of my work. Mr. Parker also read 

 the earlier portions of my manuscript. 



The material was prepared by ordinary histological methods ; but in- 

 asmuch as many of the hardening reagents and stains which I tried gave 

 thoroughly unsatisfactory results, I may state in brief the treatment 

 which proved most successful. The embryos of both Eana and Bufo 

 can be satisfactorily killed in Kleinenberg's picrosulphuric mixture ; 

 they can then be successfully stained in Orth's lithium-picrocarmin. 

 The object should be exposed to the action of the stain as long as possible, 

 care being taken to guard against maceration. In order to accomplish 

 this purpose, it has frequently proved advantageous to stain the object 

 twice, removing it after the first staining to strong alcohol. In passing 

 the stained object through grades of alcohol, it is important to keep a 

 little picric acid dissolved in the several fluids in order to prevent the 

 alcohol from extracting the yellow stain from the specimen. Embryos 

 treated in this way show a very effective double stain. The nuclei are 

 bright carmine, contrasting with the yellow color imparted by the picric 

 acid to the yolk spherules among which they are found. As a killing 

 reagent, Merkel's fluid also gives good results. It should be followed 

 by Kleinenberg's hematoxylin, and the decolorizing should be watched 

 with care. 



With Amblystoma the best method of treatment is that with Fol's 

 chromic-osmic-acetic mixture, followed by Czokor's cochineal. The 

 picrosulphuric mixture followed by picrocarmin, as recommended for 

 liana and Bufo, is also of service. 



It is usually best to stain on the slide ; and, in my experience, satis- 

 factory results with hematoxylin can very rarely be reached by staining 

 in toto. 



II. Descriptive Part. 



In the following account of the development of the pronephros and 

 segmental duct, I shall first treat these organs descriptively. For this 

 purpose, I shall take up in succession Rana, Bufo, and Amblystoma, and 

 shall describe selected stages in the development of each. This account 

 will be followed by a general discussion of nephridial organs, in which 

 the results of other investigators will be reviewed. 



