302 CYTOLOGICAL METHODS. 



plasm as to interfere with the observation of it. So that if the latter 

 is the principal object of study, a thin fixation, one in which the 

 spongioplasm is entirely preserved, but the hyaloplasm only partly, 

 may be the better. 



The spongioplasm is the easier to fix of the two, and the majority 

 of acid fixatives will preserve it more or less. The best images I 

 have obtained are those given by liquid of Flemming or -Hermann 

 in cells in which the action of the reagent has been moderate, 

 i.e. insufficient to thoroughly fix the hyaloplasm at the same time. 

 Nearly, if not quite, as good, is Bouin's picro-formol, which has the 

 great advantage of being very favourable for plasma-staining. I 

 have also had very good results with vom Rath's picro-osmic and 

 picro-platinosmic mixtures, and with acid sublimate. 



Hyaloplasm is not nearly so easy to fix, and there are only two 

 reagents in common use that readily give a really full fixation of it ; 

 these are osmic acid and bichromate of potash. 



Osmic acid acts as a fixative of hyaloplasm in liquid of Flemming 

 or Hermann, but only gives a full fixation in the outer layers of the 

 material ; and in these it easily happens that many or most of the 

 cells are ruined by over-fixation. See 35. 



This defect may be to a certain degree corrected by taking the 

 osmic acid weaker than is usual. Thus by successively reducing 

 the proportion of this ingredient in liquid of Hermann, I have 

 found that it can be brought down to one-eighth of the prescribed 

 amount without loss of the distinctive characters of the fixation. 



The defect of want of penetration seems to be incurable. See 

 35 and 42. Substitution of more highly penetrating reagents 

 such as picric acid, for the chromic acid or platinum chloride does 

 not help in the least ; you only get the osmic fixation outside, no 

 whit deeper than before, and a picro-acetic fixation, instead of a 

 chromo- or platino-acetic one, in the deeper layers, that is all. In 

 view of these defects of osmic mixtures, it may often be advisable, 

 where hyaloplasm, or its enclosures, is the chief object of study, 

 to have recourse to bichromate of potash. The formula that has 

 given me the finest fixations is that of LINDSAY JOHNSON, but it has 

 the drawback that there is risk of osmication in the outer layers. 

 In this respect liquid of Tellyesniczky, 52, is to be preferred. 



Corrosive sublimate gives a fairly full fixation ; but I believe it 

 frequently produces serious artifacts, HEIDENHAIN'S " Lanthanin ' 

 being one of them. Heidenhain's solution, 64, containing as it 

 does some 11 per cent, of sublimate, without the addition of any 

 acid to neutralise its shrinking action, seems to me to be an inad- 



