5 8 BASIC METHODS FOR EXPERIMENTS 



I think, these, if they have value, are for in toto mounts only. 

 The addition of chromic acid to fixing reagents containing picric 

 acid I have never found as valuable for marine eggs as workers 

 have found it to be for tissue cells generally. 



Of the many fixing reagents containing picric acid by far 

 the best is Bouin's. The formula for this is as follows: 75 parts 

 saturated aqueous solution of picric acid plus 25 parts formol 

 plus 5 parts glacial acetic acid. 



After fixation with Bouin's, the eggs are removed to 80 per 

 cent, alcohol containing a few crystals of lithium carbonate with 

 repeated changes until no more yellow color tinges the alcohol. 



Chromic acid 



Fixatives containing chromic acid are Zenker's, Tellye- 

 sniczky, Flemming, etc. Zenker's I do not recommend for 

 marine eggs. Tellyesniczky is good in some cases. Its formula 

 is as follows: 



Potassium bichromate 3 grms. 



Glacial acetic acid 5 cc. 



Water 100 cc. 



Since 1910 I have used a modification of Tellyesniczky, 

 which consists of the addition of 10 cc. of formol to 90 cc. of 

 Tellyesniczky 's solution. In this case, the formol is added 

 immediately before the fixative is used. 



Osmic acid 



Many competent cytologists do not employ osmic acid 

 because of some very serious drawbacks to its use. Osmic acid 

 in fixing solutions is notoriously poor in power of penetration. 

 It is often a serious hindrance to staining. Where it is allowed 

 to act too long, it produces what is spoken of as "over-osmifi- 

 cation"; such sections give a blackened and muddy appearance. 

 In my experience, these obstacles may be overcome. 



In the first place, one may use osmic acid alone or together 

 with other reagents with assurance of perfect results if one takes 

 objects of sufficiently small size. Thus it need not fail when 

 used on the minute eggs of marine animals. Blocks of tissue 



