310 EMBRYOLOGICAL METHODS. 



acid, a solution of 0*1 per cent, for twenty -four hours, followed 

 by a solution of 0*3 per cent, for twenty-four hours more, then 

 by 70 per cent, alcohol for a day, 90 per cent, alcohol for two 

 days, and lastly absolute alcohol. They also recommend a 0'5 

 per cent, solution of osinic acid, in which the embryo remains 

 for two hours and a half in the dark, and after washing is 

 brought into absolute alcohol. 



HENNEGUY prefers the osmic acid and alcohol mixture of 

 Ranvier and Vignal, or Flemming's mixture followed by 

 successive alcohols. 



Staining and imbedding may be performed by the usual 

 methods. 



Up to about the fiftieth hour embryos may be mounted 

 entire, in glycerin or balsam. 



592, M, Duval's Orientation Method (Ann. d. Sc. nat. ZooL, 

 1885). In the early stages of the development of the ova of 

 Aves, before the appearance of the primitive streak, it is 

 difficult to obtain a correct orientation of the hardened cica- 

 tricula, so as to be able to make sections in any desired 

 direction. Duval, starting from the fact that during incuba- 

 tion the embryo is almost always found to be lying on the 

 yolk in such a position that the big end of the egg is to the 

 left, and the little end to the right of it, marks the position 

 of the blastoderm in the following way. 



With a strip of paper 5 millimetres wide and 50 millimetres 

 long you construct a sort of triangular bottomless box. You 

 lay this on the yolk, enclosing the cicatricula in such a posi- 

 tion that the base of the triangle corresponds to what will be 

 the anterior region of the embryo, and its apex to the posterior 

 region ; that is to say, if the big end of the egg is to your 

 left, the apex of the triangle will point towards you. You 

 now, by means of a pipette, fill the paper triangle with 0'3 

 solution of osmic acid. As soon as the preparation begins to 

 darken you put the whole egg into weak chromic acid, remove 

 the white, and put the rest into clean chromic acid solution 

 for several days. After hardening you will find on the sur- 

 face of the yolk a black triangular area, which encloses the 

 cicatricula and marks its position ; you cut out this area with 

 scissors and a scalpel, and complete the hardening with 

 chromic acid and alcohol. 



