PRACTICAL INSTRUCTIONS. 465 



water). The excess of this fixative must be removed by repeated 

 changes of 50 per cent, and then 70 per cent, alcohol. 



For general purposes when histological details are not 

 required, Perenyi's fluid is useful. It fixes rapidly, and the 

 specimen can be transferred direct to 70 per cent, alcohol. Be* 

 fore staining in bulk, remove all traces of acid by thoroughly 

 washing in alcohol or water. 



For making sections of the earlier stages {segmentation 

 and up to twelve hours' incubation) an osmic acid mixture, such 

 as Flemming's strong mixture (see Appendix), is advisable. 

 Fixation will be complete in five or ten minutes. The tissues 

 should then be well washed in water and the sections prepared 

 as soon as possible. 



Another excellent fixative is corrosive-acetic mixture (v. 

 Appendix). The sections should be treated with a weak solution 

 of tincture of iodine before staining. 



Preparation of Preserved Embryos. 



In order to preserve the embryo, the blastoderm must be 

 removed from the yolk. This operation requires care, and 

 two dangers have to be particularly avoided : (1) the crump- 

 ling of the blastoderm ; (2) the rupture of the yolk. In order 

 to proceed successfully some operators take the whole yolk out 

 of the egg and out of the solution, and then place a ring of 

 gummed paper round the blastoderm. When this has adhered 

 a circular cut is made along the outer edge of the paper, the 

 egg is returned to the salt solution, and the blastoderm floated 

 off into a watch-glass or slide. It is better, however, to proceed 

 as follows : Cut round the outer margin of the blastoderm, 

 after making a window in the upper region of the egg. Lift 

 the blastoderm and some salt solution carefully on to a watch- 

 glass, remove any adherent yolk, and straighten out any folds 

 by sucking with a pipette. The vitelline membrane should be 

 removed from the blastoderm before the fixing fluid is added. 

 If the above instructions are carefully performed, the blasto- 

 derm separated from the yolk will be found adhering to the 

 vitelline membrane. In order to separate them, hold the vitelline 

 membrane by forceps with its blastoderm surface downwards 



