36 LNTERTEBRATA CHAP, n 



sections be examined from time to time under a microscope as 

 the stain is being removed, a point will be detected at which 

 the whole section takes on a reddish colour, and the nuclei stand 

 out prominently. When this is observed the sections should be 

 washed free from the acid alcohol by immersing them in 70 per 

 cent alcohol. They should then be held inverted for a few moments 

 over the mouth of a bottle of strong ammonia, the escaping fumes 

 from which neutralize the last traces of acid, and the sections, now 

 of a beautiful blue colour, may be dehydrated and mounted. The 

 different tissues yield up their stain in different degrees, and a 

 beautiful differentiation is effected by the different tints of blue. 



If a double stain be desired it can be effected by finally dehydrat- 

 ing the sections in absolute alcohol to which eosin has been added, 

 but if thsy remain too long in this solution all the haematoxylin 

 will be washed out. 



The methods described in this chapter are general methods 

 applicable to all classes of embryos ; special methods will be described 

 in the chapters dealing with special groups. 



