318 EMBBYOLOGICAL METHODS. 



branes is best avoided. Membranes should either be dissected 

 away or left in situ, and cut with the rest of the egg, accord- 

 ing to the nature of the case. The great obstacle to section 

 cutting is the brittleness of the yolk. This difficulty may be 

 overcome as follows : After fixing and treating with alcohol, 

 prick the chorion and stain with borax-carmine. Put the 

 stained ova for twelve hours into a mixture containing 20 c.c. 

 of 70 per cent, alcohol, one drop of concentrated hydrochloric 

 acid, and a knife-pointful of pepsin (it is not necessary that 

 all the pepsin should be dissolved). The ova may then be 

 treated with alcohol, oil of bergamot, and paraffin, and (with 

 some exceptions, amongst which is Bombyx mori) will be found 

 to cut without crumbling. 



The contents of fresh ova may conveniently be studied by 

 means of the following fluid : 



Distilled water . . . .80 c.c. 



Glycerin . . . . . . 16 



Formic acid . . . . 3 



1 per cent, osmic acid . . . 1 

 Dahlia 0'04 grm. 



The eggs are simply teased in a drop of the liquid, and a 

 cover-glass put on. If it be desired to preserve the prepara- 

 tion, nothing more is necessary than to lute the cover-glass. 



618. Lepidoptera (BOBRETZKY, Zeit. f. tciss. Zool., 1879, p. 

 198). Ova (of Pieris cratsegi and Porthesia chrysorrhoea) are 

 slightly warmed in water and put for sixteen to twenty hours 

 into 0'5 per cent, chromic acid. The membranes can then be 

 removed, and the ova brought for a few hours into absolute 

 alcohol, stained with carmine, and cut. 



619. Blattida (PATTEN, Quart. Journ. Mic. Sd., 1884, p. 549) . 

 The ova or larvae are placed in cold water, which is gra- 

 dually raised to 80 C. You leave off heating as soon as the 

 ova have become hard and white. Pass very gradually 

 through successive alcohols, beginning with 20 per cent. ; 

 stain with Kleinenberg's haematoxylin or Mayer's cochineal 

 (only alcoholic stains will traverse the chorion). The ova 

 may remain in the haematoxylin for five or six days, and be 

 washed out in alcohol containing one drop of HC1 per 20 

 grms., in which they should remain for several days, and then 



