EMBRYOLOGY OF CRYPTOBRANCHUS 93 



Fix for several days; preserve in 70 per cent or 80 per cent 

 alcohol. 



This mixture is especially useful for the yolk-laden ovarian 

 eggs, and for maturation stages; it is not very satisfactory for 

 embryonic stages. Envelopes, if present, must be removed before 

 the eggs are fixed in this solution. The best results are obtained 

 by sectioning the material soon after preservation. 



Solution E: Zenker's. This mixture was found most useful 

 for the early stages of ovogenesis, before the formation of any 

 considerable amount of yolk. It is not good for embryonic stages, 

 unless parts of the embryo are to be dissected off from the yolk 

 before sectioning. It gives very inferior preparations for sur- 

 face study in every stage. 



For the early stages of ovogenesis, before the formation of yolk, 

 both Flemming's and Bouin's solutions were used with fair results. 

 For larvae after the disappearance of the yolk sac, Tellyesnicky's, 

 Zenker's, or almost any good fixing solution may be used. 



Of the various mixtures experimented with for the yolk-laden 

 stages, those containing picric acid proved to be the very worst. 

 The invariable result of the use of a solution containing picric 

 acid was to cause the egg to disintegrate. 



In preserving the embryological material of Necturus, Solution 

 B was principally used. In the early stages of development, 

 before the formation of the neural folds, the results are not so 

 uniformly good as with Cryptobranchus ; this is perhaps due to 

 the fact that in these stages the eggs of Necturus are almost 

 necessarily preserved before the removal of the very closely-fitting 

 gelatinous envelopes. In successfully preserved eggs in the 

 cleavage stages, the furrows of the upper hemisphere are more 

 conspicuous and the contour of the micromeres more rounded, 

 than in Cryptobranchus; they are thus, except for difficulties 

 arising from the character of the envelopes, more favorable objects 

 for photography. Late gastrula and neural groove stages of 

 Necturus, preserved by this method, are rarely so favorable for 

 surface study as the same stages in Cryptobranchus. After the 

 formation of the neural folds, when a space has appeared between 

 the envelope and the egg, the embryos of Necturus are preserved 



