94 



acid, to saturation. This is a valuable embryological fixer, especially service- 

 able with eggs that have a good deal of yolk. 



Fix for 4-12 hours, transfer to 67% alcohol for i day, 82% alcohol, several 

 days. It is necessary to wash the tisBue well in several changes of the alcohol, 

 as the picro-nitric is washed out with diflSculty. 



§ 37. Picro-sulphuric. Formula : Distilled water, 100 c.c. ; sulphuric 

 acid, 2 c.c. ; picric acid, to saturation. 



This may be used full strength or diluted with 1-3 volumes of distilled 

 water. It is a embryological fixer, useful for many eggs, chiefly invertebrate, 

 and for many invertebrate animals. In general, fix for 1-6 hours, remove to 

 f^% (70%) alcohol for a day, and place in &2% alcohol, in which it should 

 remain until most or all of the yellow color has been extracted. 



\ 38. Nitric acid. A 10% solution of nitric acid is serviceable in fixing 

 the blastoderm of the chick. 



SECTIONING. 



§ 39. In addition to the examination of tissue by the separa- 

 tion of the component elements — isolation — it may be examined 

 microscopically after cutting very thin slices or sections of it. This 

 may be done free-hand or by means of a special machine, a micro- 

 tome, and with or without an imbedding and supporting mass. 



For the finer work an imbedding mass and a microtome must 

 be used. Free-hand sectioning without an imbedding mass, and 

 even without previous hardening, is, however, necessary or advis- 

 able when economy of time is a desideratum, as in clinical exam- 

 inations of tissue, when one wishes to study the part alive or fresh 

 (i. e., not treated with reagents), or if the reagents necessary for 

 fixing and imbedding destroy or alter the structural features to be 

 investigated. 



The ability to recognize tissues and organs unaffected by re- 

 agents and without the employment of methods involving the ex- 

 penditure of time and effort is very desirable, especially in patho- 

 logical work, when haste often forbids the employment of the finer 

 methods, were facilities for their use available., as in many cases 

 they are not. Great skill in the use of simple tools may be gained 

 and counts for much. It should be remembered also that the 

 greater one's knowledge of a structure the less the need to resort to 

 special methods of preparation for its recognition. 



