GOLGI BODIES ETC. 301 



depend for their application on the use of complicated fixing and 

 staining methods, are to be used cautiously. For example, 

 Benda (§ 697) and Altmann-Bensley methods (§§ 694 and 701) 

 will stain granules other than mitochrondia, while the Cajal 

 formalin uranium and silver nitrate technique impregnates 

 bodies apart from the Golgi apparatus. In all these cases, however, 

 the number of exceptions is small, and suitable differentiation 

 between two doubtful bodies can be made by some other method. 



Over-impregnations or impregnations done with disintegrating or 

 unsuitable osmic or silver nitrate fluids produce preparations in which 

 not only the mitochondria but even cell walls and nuclei are more or less 

 blackened. It would be incorrect to state that only experienced tech- 

 nicians are able to work these methods so as to get the degree of 

 specificity expected by those who use these methods for research work. 

 In molluscan gonads, the tissues of young mammals, insect gonads, 

 etc., perfectly good preparations may be made by beginners, and the 

 degree of specificity (which is quite considerable) judged. 



687. On Killing Animals for Cytological Purposes. So far as 



possible avoid narcotics of any sort. Either cut off the heads of 

 invertebrates, or, if delicate like some worms, drop them whole 

 in the fixative ; kill vertebrates by a blow on the head, or by 

 pithing. If for a study of brain, bleed, or anaesthetise in coal gas, 

 less preferably chloroform or ether. Insects can be killed with 

 cyanide or xylol. 



Hints on removing Tissues and Cutting. Avoid pinching the 

 material with forceps, as this is said to introduce artifacts ; it is 

 preferable to remove tissue without recourse to dissection under 

 tap-water or salt solution ; for Kopsch techniques, quickly 

 remove blood or lymph, etc., from surface of material with aq. 

 dest. before placing in fixer ; for cutting tissue the best instrument 

 is a new safety razor blade stuck in a special holder made for the 

 purpose, or in a split penholder, or held by artery forceps. When 

 working on arthropods, it is best to dissect the organ from the 

 animal, instead of preserving the whole body ; surrounding fat, 

 etc., should be removed. See also §§10 and 628. 



688. Washing Out for Cytological and Histological Purposes. 

 The beginner is often puzzled by the problem of how long to wash 

 out after fixatives. Elsewhere (§ 35) we have mentioned that 

 usually picric and corrosive material is washed out in alcohol, 

 whereas chrome fixed tissue is treated with tap-water. This is 

 not all, however, and the washing out tvill often he found to have 

 important results on the subsequent staining ; e.g. small animals 

 like daphnids, fixed in Champy's fluid overnight, must only be 

 washed out a few minutes, or under an hour. If such washing is 

 prolonged too much, the subsequent hajmatoxylin stain will be a 

 failure, because it seems necessary to leave some chrome-osmium 

 in the tissues to make staining a success (§ 706). Larger objects, 



