276 CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 



many insects, or short sections of chromosomes which remain 

 condensed (heteropycnotic) during the interphases of mitosis, or 

 possibly, are formed in other ways. Such structures stain as 

 chromatin with the common nuclear (basic) dyes, and give a 

 positive reaction to Feulgen's stain following the fixatives 

 ordinarily used for the preservation of chromosomes. The 

 " plastin," of which plasmosomes are wholly or mostly composed, 

 is acidophil and does not stain with Feulgen's method. Some 

 nucleoli are compound in character and contain both chromatin 

 and plastin. 



Although our knowledge of nucleoli is still incomplete, the 

 following facts seem well established with regard to them : 

 (1) While typical chromatin nucleoli stain with basic dyes and 

 plasmosomes with acid stains, under certain conditions not well 

 understood, i.e., during spermateleosis of the sperm head, 

 chromatin may take acid dyes. It is also well known that plas- 

 mosomes may be made to react differently to a specific dye 

 depending both on the nature of the fixative agents and the _pH. 

 For example, in plant cells fixed with Bouin's fluid, the plasmo- 

 some stains intensely with iron ha?matoxylin and holds this dye 

 after long extraction. If Navaschin's fluid is used for fixation, 

 however, it shows little affinity for this stain. (2) It has been 

 shown repeatedly that plasmosomes may arise from, or in intimate 

 association with, a definite region of a specific chromosome. In 

 some instances the connection between the two persists. This 

 being so, we might expect that bodies made up of plastin might 

 show traces of chromatin, and vice versa. (3) There is no reason, 

 a priori, to suppose that plasmosomes are made up of only one 

 sort of material . It is possible that diverse acidophilous substances 

 appear as plasmosomes. 



Nucleoli are commonly studied by three diverse methods. One 

 involves reactions to dyes alone, the second consists of impreg- 

 nating with silver or osmium, and the third involves differences in 

 solubility of nuclear components. 



Nucleoli are comparatively easily preserved and any of the 

 standard chromosome fixatives may be used. Chromo-aceto- 

 osmic mixtures are the best, perhaps, because of the ease with 

 which some double stains follow this fixation. To distinguish 

 between the chromatin and plastin the most satisfactory method 

 we have at present is Feulgen's stain followed by some counter- 

 stain, such as light green. If this method is to be followed, 

 select one of the fixatives which lends itself to this test. See 

 § 623 for further details. Other tests are given in a paragraph 

 below. For staining nucleoli the principal stains which are 

 commonly employed are : safranin and methyl green, iron 

 hsematoxylin (after strong Flemming fixation), Auerbach's methyl 

 green and acid fuchsin (this does not work well after fixation 



