CHAPTER XXVI. 317 



of Cajal, which consist of silver nitrate impregnation following 

 formalin fixation. Intm-vitam methods, such as janus green, 

 neutral red, or dahlia violet are also used extensively. The mito- 

 chondria are extremely fuchsinophile, and after chrome-osmium 

 fixation stain strongly in iron-alum haematoxylin. The Golgi 

 apparatus of somatic cells and of ovarian cells rarely stains by these 

 methods (Altmann or Heidenhain) unmodified, although the Golgi 

 apparatus of the male germ cells nearly always stains in fuchsin or 

 hsematoxylin after chrome-osmium or formalin fixation. Besides 

 observing the Golgi apparatus and mitochondria, certain workers 

 have turned their attention to the study of fats and yolk (vitellus) 

 in cells. 



In 768 is a special article on fats and lipoids, and on methods for 

 their study ; on the following pages are set forth various techniques 

 for the investigation of definite cell organs known to be partly lipoid 

 in nature. The application of all these methods to embryological 

 study opens the way to a valuable field for research. In 768 will 

 be found definitions of the terms ' fat," ' lipoid," and " lipin." 

 Fats or lipoids form a special part of almost all cell-organs, as seems 

 to be indicated by fixing tests, and so far as we know such substances 

 are always intimately associated with protoplasm. Many of the 

 lipoids appear to be able to form with certain metallic salts 

 or oxides such as Cr0 3 , K 2 Cr 2 7 , PtCLt, OsO^, etc., compounds 

 insoluble or only slowly soluble in alcohol or such clearing oils as 

 xylol, benzole, or chloroform ; this is one of the several reactions 

 which take place when a cell is fixed in such a fluid as that of 

 Flemming (without acetic acid), Champy, or Altmann, and subse- 

 quently dehydrated and cleared. See 29, 30 and 31. 



674. Choice of Method.* We have given below a number of 

 methods for lipoid granules, mitochondria, and other cell inclusions, 

 and not all are suitable for every piece of work. It is very rare to 

 find that one single method will produce the same good result in 

 both vertebrate and invertebrate tissues. In the same way, methods 

 which act satisfactorily with amphibia will often give disappointing 

 results with mammalia. Osmic-chrome fixation will nearly always 

 be found excellent for all classes of invertebrata ; Flemming- without- 

 acetic acid and Champy-Kull can be highly recommended. For 

 amphibia the addition of some K 2 Cr 2 07 to the Flemming is necessary 

 before a correct fixation of the mitochondria is obtained ; thus 



* The beginner is recommended to master such techniques as those of 

 Bourn's fluid and borax carmine, or Zenker and Ehrlich's hsematoxylm 

 before trying these methods. 



