264 EMBRYOLOGIOAL METHODS. 



may be referred. What follows here is given merely as being 

 of more recent publication. 



If it be desired to observe a living embryo by transmitted 

 light, the egg should be opened under salt solution, as described 

 in Foster and Balfour. A little of the white is then removed 

 through the window, the egg is lifted out of the liquid, and a 

 ring of gummed paper is placed on the yolk so as to surround 

 the embryonic area. As soon as the paper adheres to the 

 vitelline membrane, which will be in a few minutes, a circular 

 incision is made in the blastoderm outside the paper ring. 

 The egg is put back into the salt solution, and the paper ring 

 removed, carrying with it the vitelline membrane and the 

 blastoderm, which may then be brought into a watch-glass or 

 on to a slide and examined under the microscope (DUVAL). 



556a. Gerlach's Window Method (Nature, 1886, p. 497 ; Journ. 

 Roy. Hie. Soc., 1886, p. 359). Kemovewith scissors the shell 

 from the small end of the egg : take out a little white by means 

 of a pipette ; the blastoderm will become placed underneath 

 the window thus made ; and the white that has been taken out 

 may be replaced on it. Paint the margins of the window with 

 gum mucilage, and build up on the gum a little circular wall 

 of cotton wool ; place on it a small watch-glass (or circular 

 cover-glass), and ring it with gum. When the gum is dry, 

 the cover is further fixed in its place by means of collodion 

 and amber varnish, and the egg is put back in its normal posi- 

 tion in the incubator. The progress of the development may 

 be followed up to the fifth day through the window. 



A description of further developments of this method, with figures of 

 special apparatus, will be found in Anat. Anz., ii, 1887, pp. 583, 609 ; see 

 also Zeit.f. wiss. Mile., iv, 3, 1887, p. 369. 



557, Preparation. During the first twenty-four hours of in- 

 cubation, 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 ad- 

 vantageous. The egg should be opened in salt solution, then 

 lifted up a little, so as to have the blastoderm above the sur- 

 face 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 



