E 31 BRYOLOGY 375 



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 mem- 

 brane and the blastoderm, which may then be brought into a 

 watch-glass or on to a slide and examined under the microscope 

 (Duval). 



Gerlach's Window Method (Nature, 1886, p. 497). Remove with 

 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 just 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 

 position 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 Paton, Journ. Exper. ZooL, xi, 1911, p. 469 (cultivation of 

 the embryo in vitro). 



817. 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. 

 Andrews (Zeif. wiss. Mik., xxi, 1904, p. 177) separates the 

 blastoderm at this stage by injecting picro-sulphuric acid (not 

 any rapidly acting fixative) firstly, between the blastoderm 

 and the vitelline membrane, so as to separate the two above, 

 and then between the blastoderm and the yolk, so as to free the 

 blastoderm below and float it up. This done, the membrane may 

 be incised and the blastoderm removed. The injection is best 

 done with a pipette having a fine point bent upwards. 



In later stages, when the embryo is conspicuous, the blastoderm 

 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 blasto- 

 derm is then treated with some fixing solution dropped on it from 

 a pipette (1 per cent, solution of osmic acid, or Ranvier and 

 Yignal's osmic acid and alcohol mixture, iodised serum, solution 

 of Kleinenberg, 10 per cent, nitric acid, etc.). 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 

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



