136 EZRA ALLEN 



thin sections did not add materially to the ease of studying the 

 individual chromosomes, but were found useful in making some 

 of the photographs, as noted under explanation of figures. As 

 nuclear stains, iron-alum-haematoxylin and safranin were em- 

 ployed; the counterstains were orange G., methyl green, and 

 acid fuchsin. Flemming's triple stain does not yield very satis- 

 factory results after any picro-formol-acetic fixation unless the 

 slides are soaked in 50 per cent hydrogen peroxide from five to 

 ten hours; but even with this treatment I did not find the stain 

 very satisfactory upon the rat testis. The most satisfactory stain 

 for chromosomes, whether used after Flemming's fluid or the 

 picro-formol-acetic-chromic-urea fluid (described in my paper to 

 which reference has been made as B-15), was iron-alum-haema- 

 toxylin without counterstain. 



Acknowledgments. The writer desires to acknowledge his great 

 indebtedness to Professor C. E. McClung for constant advice, 

 criticism, and encouragement, and to the University of Pennsyl- 

 vania and The Wistar Institute for laboratory facilities and 

 supplies. 



3. OBSERVATIONS 



The Structure of the Rat Testis 



The testis of the rat is typical of the mammals. In the 

 adult, each tubule is constantly active in the production of 

 spermatozoa up to old age. This production of the mature 

 spermatozoa is a wave-like activity passing along the tubule, 

 so that in one part of the wave dividing spermatogonia 

 and nearly mature spermatozoa appear, while in another part 

 early stages of first spermatocytes and spermatids occur. The 

 latter condition is shown in figure 1. In other parts other com- 

 binations of the various stages are to be seen. Von Ebner ('88) 

 worked out this cycle in the rat, finding it to occupy 32 mm. in 

 length. That is, in a given tubule, dividing spermatogonia are 

 found in localities separated by 32 mm., of the tubule length. 

 Similarly, first spermatocyte metaphases, or other definite stages, 

 repeat themselves along the tubule at this distance, and in each 

 case the other cells show their corresponding stages. Two por- 

 tions of the wave are shown in figures 1 and 46. 



