266 EMBRYOLOGIOAL METHODS. 



tion. Duval, 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 poste- 

 rior 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, re- 

 move 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. 



Another way of hardening is to place the egg, after the action of the 

 osmic acid, in a solution of chromic acid which is then raised to boiling- 

 point on a water-bath ; after cooling, the blackened region is cut out, and 

 the hardening completed in the usual way with chromic acid and alcohol. 



559a. Keller's Method (Arch.f. mik. Anat., xx, 1881, p. 182). Chromic 

 acid, O'l per cent., twenty-four hours; ibid., O2 per cent., twenty-four 

 hours ; and so forth, with daily increments of O'l per cent, up to O5 per 

 cent. When hard, remove the blastoderm together with a segment of the 

 yolk. Water, twenty-four hours. Stain, and imbed. 



Reptilia. 



560. General Directions. The methods described above for 

 the embryology of birds are applicable to the embryology of 

 reptiles. During the early stages the blastoderm should be 

 hardened in situ on the yolk; later the embryo can be 

 isolated, and treated separately with Kleinenberg's solution 

 and alcohol (STRAHL, Arch.f. Anat. u. Phys., 1881, p. 123). 



561. Kupffer's Method (Ibid. 1882, p. 4). The ova are opened and 

 the albumen removed under osmic acid of -^ per cent. The yolk is put 



