EMBRYOLOGICAL METHODS. 339 



convex surface of a watch glass, and leave it so during- the 

 hardening. 



For hardening, HENNEQUY prefers the osmic acid and alcohol 

 mixture of Ranvier and Vignal, or Flemming's mixture fol- 

 lowed by successive alcohols. 



Stain and imbed by the usual methods. 



Up to about the fiftieth hour embryos may be mounted 

 entire in glycerin or balsam. 



594. M. DUVAL'S Orientation Method (Ann. Sc. Nat., 1884, 

 p 3). I n 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 direc- 

 tion. DDVAL, starting from the fact that during incubation 

 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 

 per cent, 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 surface 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. 



See also the method of HIROTA, Journ. Roy. Hie. Soc., 

 1895, p. 118. 



595. KIONKA'S Orientation Method (Anat. Hefte, 1 Abth., 

 iii, 1894, p. 414; Zeit. f. ids*. Mik., xi, 2, 1894, p. 250).- 

 Open the egg under salt solution, free it from the shell and 



