92 BERTRAM G. SMITH 



During the process of washing in formahn the color changes 

 from yellow to green. The yolk becomes dark green, while the 

 blastodisc or embryo proper is much lighter in color, giving a 

 sharp differentiation of the protoplasmic portions of the egg. The 

 form of the egg is preserved perfectly, and remarkably good defini- 

 tion for surface study is secured. The eggs are easily sectioned 

 by the paraffin method. 



Not until after the closure of the neural folds is it possible to 

 alter the proportions in the formula as given above without 

 injury to the form of the embryo; an increase in the proportion of 

 potassium bichromate results in the collapse of the embryo when 

 in melted paraffin if not in an earlier stage of the process of prepa- 

 ration for imbedding. For later stages the proportion of potassium 

 bichromate may be shghtly increased (e.g., to 1^ per cent), with- 

 out detriment to the surface features and perhaps with some gain 

 in the histological results. 



Solution C. Sublimate-acetic-formalin. 



Saturated solution corrosive sublimate in 10 per cent formalin . . . 971 parts 

 Glacial acetic acid 2| parts 



Fix for a few hours, then transfer to formalin for a few days to 

 insure thorough fixation of the yolk. Wash and preserve in either 

 formalin or alcohol. 



This is not so satisfactory a fixing solution as Solution B, but 

 may be used for comparison. For surface study the results, 

 especially in the early stages, are decidedly inferior to those 

 secured with Solution B. For sectioning, good results are se- 

 cured in the early cleavage stages and after the closure of the 

 neural folds; the mercury crystals must be removed by prolonged 

 treatment with iodin. In the blastula and gastrula stages the 

 embryo usually collapses during the process of preparing for im- 

 bedding. 



Solutio7i D: Lavdowsky's. 



Formalin 10 parts 



Alcohol, 95 per cent 50 parts 



Glacial acetic acid 2 parts 



Water 40 parts 



