APP.] STAINING EMBRYOS. 429 



this is to stain the embryo as a whole, rather 

 than to stain the individual sections after they 

 have been cut. 



We have found hsematoxylin and borax- 

 carmine the best reagents for staining embryos 

 as a whole. 



a. With hsematoxylin. 



The best solution of hsematoxylin, one for 

 which we are indebted to Kleinenberg, is made 

 in the following way. 



(1) Make a saturated solution of crystallized cal- 

 cium chloride in 70 p.c. alcohol, and add 

 alum to saturation. 



(2) Make also a saturated solution of alum in 70 

 p.c. alcohol, and add 1 to 2 in the proportion 

 of 1 : 8. 



(3) To the mixture of 1 and 2 add a few drops of 

 a saturated solution of hsematoxylin in ab- 

 solute alcohol. 



(4) It is often the case that hsematoxylin solution 

 prepared in this way has not the proper 

 purple tint ; but a red tint. This is due to 

 acidity of the materials used. The proper 

 colour can be obtained by treating it with 

 some alkaline solution. "We have found it 

 convenient to use for this purpose a saturated 

 solution of sodium bi-carbonate in 70 p.c. 

 spirit. (The exact amount must be deter- 

 mined by experiment, as it depends upon the 

 amount of acid present.) 



The embryo shovild be placed for some hours 

 in absolute alcohol, before staining with has- 



