ON EGGS OF MARINE ANIMALS 69 



melting at 6o°C. in no wise impairs the quality of the fixed egg. 

 The notion that this temperature injures the already dead egg 

 is in my judgment wholly illusory. So too, the belief that 

 celloidin is superior to paraffin as an imbedding medium-. 



I have made sections of two lots of eggs in the same stage of 

 development, one lot imbedded in celloidin and the other in 

 paraffin. Careful study revealed that the eggs imbedded in 

 celloidin were in no wise superior to those imbedded in paraffin. 

 The cumbersome, tedious method of celloidin-imbedding cer- 

 tainly has advantages over paraffin in the case of some objects; 

 these do not obtain in the case of the minute eggs under discus- 

 sion. Indeed, I find that even for some objects refractory to 

 paraffin imbedding, one need not have recourse to celloidin: 

 Johnstone's rubber-asphalt-paraffin mixture serves exellently. 

 Also, the use of chloroform or of cedar-wood oil for clearing — the 

 second followed by toluene or xylene — ^will lessen some of the 

 difficulties encountered in imbedding in paraffin and so obviate 

 recourse to celloidin. 



The trick, if it may be called such, in using paraffin for 

 imbedding eggs at the sea-side, is to employ, I repeat, glassware 

 of absolute cleanliness and to avoid moisture and over-dryness 

 (from the xylene or toluene) of the eggs when subjected to 

 paraffin infiltration. Manipulate with quickness and deftness 

 in order to prevent excessive exposure which again increases the 

 chances of water being introduced into the paraffin mass. 



Not one single air bubble should be allowed to remain in the 

 paraffin mass; to this end, prick with a hot needle any such 

 bubbles. Also, the paraffin must not be allowed to crystallize. 

 Either air bubble or crystals in the paraffin block render good 

 sectioning difficult. The cooled paraffin block should be per- 

 fectly transparent. 



This method of imbedding animal eggs of minute size is the 

 simplest that I know. If one prefers a more complicated method, 

 one has the choice of several. But I think that the results will 

 not be superior. Indeed, I have tried several methods, described 

 in the literature, and have used the best forms of paraffin baths 

 available. I have never had results any better than those 

 obtained with an ordinary copper plate heated with an alcohol 

 lamp. Used with care this is as good as the electric hot-plate 



