290 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



followed by treatment with iodine and then by staining with fuchsin, 

 was very valuable ; and for the same purpose the double stain of safra- 

 niu and gentian violet was of service. As microchemical tests, various 

 mixtures of acid and basic colors were employed, the most useful being 

 the Auerbach mixture of methyl green and acid fuchsin, the Biondi- 

 Heidenhain mixture, and a combination of safranin and Lichtgriin. 



The work was at first prosecuted entirely on sections, but I was soon 

 fortunate enough to perfect a method of isolating the cells, which proved 

 of the greatest service. Briefly, it was as follows : a slide was thinly 

 coated with albumen-water fixative, a small fragment of the gonad tissue 

 placed upon it in a drop of 70 per cent alcohol and immediately cov- 

 ered ; the cover glass rapidly revolved until the tissue was entirely 

 fragmented, then removed, and the slide plunged into a tube of 95 per 

 cent alcohol. This of course coagulated the albumen, and in the more 

 successful preparations nearly all of the cells were found isolated, 50 per 

 cent, or even more, being entire, at least so far as their nuclei were con- 

 cerned. The method is made possible by the brittleness of material 

 fixed in osmic acid solutions, and it offers no difficulties, success depend- 

 ing chiefly on the speed of manipulation ; while subsequent staining is 

 simple and extremely rapid. The only precaution to be taken is against 

 excessive pressure or prolonged drying, either of which causes distortion 

 of the nuclear contents. In such a study as this, the value of a satis- 

 factory method of isolating the germ cells, especially after fixation, can 

 hardly be overestimated. The advantage of studying the complex chro- 

 matin structures on entire nuclei, rather than on sections, — with the 

 accompanying necessity for reconstruction, — is self-evident ; the chances 

 of unavoidable error are so much reduced by this device that I feel as 

 though any value this memoir may possess is very largely due to the 

 adoption of this simple means of study. It was used in studying both 

 spermatogenesis and oogenesis ; but owing to the difficulty of identifying 

 the cells in the " crushing " method, the examination of the mitoses of 

 somatic cells and of oogonia was made entirely on sections, and these were 

 of course constantly employed as a check upon the other observations. 



III. Historical Survey. 



1. The Somatic Nucleus. 



Although the somatic nucleus was observed in a coelenterate as early 

 as 1872, by F. E. Schultze, it was not until 1879 that Korotneff detected 

 the nucleolus, nuclear network, and granular ground substance. Pfitzner 



