6o BASIC METHODS FOR EXPERIMENTS 



of formalin, mercury bichloride, picric acid, or in fixatives con- 

 taining these chemicals. One loses eggs on transferring them, 

 in washing and in dehydration. Any solution which contains 

 osmic acid insures the worker that he may retain every single 

 egg fixed. The osmic acid so increases the specific gravity of 

 the eggs that they settle very rapidly in all of the stages of 

 washing, dehydration and clearing. In paraffin, eggs having 

 been fixed in most solutions tend to float on the surface. They 

 scatter widely and settle slowly, sometimes not at all. One 

 never has this trouble in imbedding eggs having been fixed in 

 solutions containing osmic acid. 



When osmic acid is used in combination with chromic acid, 

 another good feature obtains. Many marine eggs are enclosed 

 in jelly. This jelly is often a source of great annoyance; in some 

 fixing solutions it never dissolves. The presence of chromic acid 

 insures the complete but harmless dissolution of the jelly. Also, 

 whereas osmic acid may be unfavorable for subsequent staining, 

 chromic acid for many stains is favorable. In some cases 

 staining capacity may be increased by treating the eggs with 

 potassium bichromate. I have also used the old method of 

 treating eggs, fixed in solutions containing osmic acid, with 

 picric acid. By and large, however, I think that the use of 

 solutions containing osmic acid in the way that I shall suggest, 

 obviates the necessity of any subsequent treatment with other 

 reagents. 



Osmic acid alone I seldom use except for fixing rapidly 

 occurring changes in the ectoplasm of eggs, as those due to 

 sperm penetration, for example, that I wish to study in the fixed 

 condition for the purpose of orienting experiments or for the 

 purpose of comparing its action with that of solutions of it in 

 combination with other reagents. For the most part I employ 

 osmic acid in a solution of chromic acid or in one of potassium 

 bichromate. 



Flemming^s solution and modifications 



Osmic and chromic acids with the addition of glacial acetic 

 acid make up several fixing solutions, Flemming's and modifi- 

 cations thereof. Thus, are available: Weak Flemming, Strong 

 Flemming, and modifications by Benda and by Meves. 



