AVES. 309 



591. Preparation. During the first twenty-four hours of 

 incubation it is extremely difficult to separate the blastoderm 

 from the yolk, and they should be fixed and hardened together. 

 In later stages, when the embryo is conspicuous, the blasto- 

 derm can easily be separated from the yolk, which is very 

 advantageous. To open the egg, lay it on its side and break 

 the shell at the broad end by means of a sharp rap ; then 

 carefully remove the shell bit by bit by breaking it away 

 with forceps, working away from the broad end until the 

 blastoderm is exposed. The egg should be opened in salt 

 solution, then lifted up a little, so as to have the blastoderm 

 above the surface of the liquid ; the blastoderm is then treated 

 with some fixing solution dropped on it from a pipette (1 per 

 cent, solution of osmic acid, or Ranvier and Vignal's osmic 

 acid and alcohol mixture, iodised serum, solution of Kleinen- 

 berg, 10 per cent, nitric acid, &c.). By keeping the upper 

 end of the pipette closed, and the lower end in contact with 

 the liquid on the blastoderm, the blastoderm may be kept well 

 immersed for a few minutes, and should then be found to be 

 sufficiently fixed to be excised. (Of course, if you prefer it, 

 you can open the egg in a bath of any fixing liquid [10 per 

 cent, nitric acid being convenient for this purpose] of such a 

 depth as to cover the yolk ; and having exposed the blasto- 

 derm, leave it till fixed [fifteen to twenty minutes] ; but I 

 think the procedure above described will generally be found 

 more convenient.) 



The egg is put back into the salt solution, and a circular 

 incision made round the embryonic area. The blastoderm 

 may then be floated out and got into a watch-glass, in which 

 it may be examined, or may be brought into a hardening 

 liquid. 



Before putting it into the hardening fluid, the portion of 

 vitelline membrane that covers the blastoderm should be 

 removed with forceps and shaking. 



In order to counteract the turning up of the edges of the 

 blastoderm that generally happens during the process of 

 hardening, it is well to get the blastoderm spread out on the 

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

 hardening. 



For hardening, FOSTER and BALFOUR recommend solution of 

 Kleinenberg for five hours, followed by alcohol. Or chromic 



