ch.x PRACTICAL EMBRYOLOGY OF THE FOWL 509 



100 c.c, common salt -75 gramme] heated to a temperature of 

 about 40° C. Submerge the egg upon its side in the salt solution and 

 remove the side of the shell which is uppermost by cutting with a 

 pair of strong scissors and then lifting off the isolated piece of shell 

 with blunt forceps. Take care to keep the point of the scissors or 

 forceps close to the inner surface of the shell so as to avoid risk of 

 injury to the true egg or " yolk." 



^ II. Egg after 42 Hours' Incubation. — Open the egg as 

 before. On removing the piece of shell the blastoderm will be 

 seen as a circular whitish area on the upper side of the yolk. Excise 

 the blastoderm by making a series of rapid cuts with the large scissors 

 through the vitelline membrane a short distance external to the 

 boundary of the blastoderm. Should the yolk happen to be tilted 

 round so that the blastoderm is not uppermost but rather at one side 

 make the first cut below the blastoderm so that the elasticity of the 

 vitelline membrane will tend to pull it upwards when the cut is 

 made. Otherwise the blastoderm may be lost by its being pulled 

 downwards. 



Having isolated the circle of vitelline membrane, with its ad- 

 herent blastoderm, slide it off the yolk by pulling gently on one side 

 with the forceps. Eemove the remains of the egg from the dish so 

 as to keep the salt solution clean. Take hold of the circle of vitelline 

 membrane at one edge with the forceps and wave it backwards and 

 forwards beneath the surface of the salt solution. The blastoderm 

 will gradually become detached. Should it not do so at once the 

 separation should be started by freeing it from the vitelline mem- 

 brane with a scalpel at one edge. Notice the difference in appear- 

 ance between the vitelline membrane and the blastoderm which has 

 been detached from it. If the blastoderm is yellow from adherent 

 yolk this should be washed off either by waving the blastoderm 

 backwards and forwards in the salt solution or by gently directing 

 jets of salt solution on the yolky surface of the submerged blastoderm 

 by a wide-mouthed pipette. 



The blastoderm should now be brought near the surface of the 

 salt solution and a watch-glass slipped under it by which it may be 

 lifted from the larger vessel. The blastoderm is so delicate that it 

 must be kept submerged in the fluid : no attempt must be made to 

 lift it above the surface by forceps. 



A microscope coverslip slightly larger than the blastoderm should 

 now be submerged in the watch-glass and the blastoderm floated 

 over it dorsal side above. The dorsal or upper side of the blastoderm 

 can easily be identified from the fact that the edges of the blasto- 

 derm tend to curl upwards. Having floated the blastoderm over the 

 coverslip the latter should be gently raised to the surface of the fluid 

 with a pair of large forceps. Take care to keep the coverslip abso- 

 lutely horizontal and lift it out of the fluid very carefully so that the 

 blastoderm is stranded on its upper surface, the lower surface of the 

 blastoderm being in contact with the coverslip. The superfluous salt 



