176 J. DUESBERG. 



obtained by leaving the pieces in the fixative for 9 hours, in the 

 silver nitrate 37 hours, and in the developer 14 hours. Instead 

 of other complicated procedures, at the suggestion of Professor 

 Cowdry I used the following method in the treatment of the 

 sections: The slides were first immersed in o.i per cent, gold 

 chloride for 2-3 hours; then in 5 per cent, hyposulphite 20-30 

 minutes. The results were excellent. Probably any nuclear 

 dye could be used as a counterstain. I resorted to Ehrlich's 

 hematoxylin, safranin, and methyl green (Cowdry, 1916), which 

 in a concentration of one half per cent., applied for 3060 seconds, 

 gives a very sharp contrast to the black color of the apparatus 

 and the pale background. For reasons to be made clear later on, 

 special methods (Mallory's, safranin-light green, and a combina- 

 tion of Congo red, iron-hematoxylin and light green) had to be 

 used for staining the connective tissue. In the application of 

 these, especially of the latter method, Professor Van der Stricht 

 kindly gave me the benefit of his wide experience. It may be 

 worth while to state in passing that Mallory's method gives just 

 as good if not better results after fixation with Flemming's 

 fluid (modifications of Benda and of Meves included) as after 

 sublimate or similar fixatives. 



Concerning the structure of the interstitial cells in the opossum ; 

 I find in the literature only two short notes; one by Whitehead 

 (1908), the other by Jordan (1911). Whitehead states that these 

 cells contain no fat. Jordan represents the mitotic division of 

 the interstitial cells and mentions their chondriosomes. Data 

 on the structure of interstitial cells in other mammals, on the 

 other hand, are abundant and will be referred to when occasion 

 arises. 



In accord with the description of a number of authors, notably 

 Reinke (1896), Lenhossek (1897), Bouin and Ancel (1903), and 

 Winiwarter (1912, i), I found both single and double-nucleated 

 interstitial cells (Figs. 2 and 7), although the latter are of far 

 less frequent occurrence. Most of the nuclei show a marked 

 depression on one side (Figs. I, 3, 5, 9 and 10); only in material 

 fixed with Regaud's fluid is this peculiarity of structure not 

 visible, probably because the nuclei swell somewhat in this 

 fixative. The process of formation of the double nuclei has not 



