114 Animal Micrology 



the head of the chick is directed away from the operator. This 

 fact affords a very reliable means of orienting the embryo, 

 especially in the very young stages when the anterior and pos- 

 terior ends are not easily recognized by the observer. 



2. Break through the shell at the broad end over the air 

 chamber by tapping it sharply and let out the air, or the broad 

 end will tilt up. 



3. Begin at the hole made in the end and with blunt forceps 

 remove the shell and shell membrane bit by bit from the upper 

 surface of the egg until the embryo comes plainly into view. 

 Remove with a pipette the thin layer of albumen which lies above 

 the blastoderm. 



4. With as little agitation of the liquid in the vessel as possi- 

 ble, by means of fine scissors cut rapidly around the blastoderm 

 just outside the vascular area. 



5. Carefully float the blastoderm into a thin watch-glass, keep- 

 ing it as flat as possible. Shake it gently to remove the piece of 

 vitelline membrane covering it, or any yolk which may adhere. 

 The aid of a needle may be necessary to remove the vitelline 

 covering. 



6. With a pipette remove all fluid from the watch-glass, leav- 

 ing the blastoderm to become dry enough to adhere to the glass, 

 but take great pains that the embryo itself does not become dry. 

 If the edges are not thus kept down they curl up and obscure 

 the embryo when the fixing fluid is added. (Some workers 

 employ a ring of paper to hold down the edges of the blastoderm.) 



7. Carefully add Gilson's fluid (reagent 15, Appendix B) until 

 the embryo is completely immersed. The fluid should be allowed 

 to act for from 2 to 3 hours. 



8. Wash in repeated changes of 50 per cent, alcohol to which 

 tincture of iodine has been added (see caution 1 under reagent 

 13, Appendix B), and stain in Borax-carmine (reagent 32, 

 chap, i) for 24 hours (Conklin's hematoxylin may be used if pre- 

 ferred; see reagent 48, Appendix B). 



9. Wash the object in water and transfer it through 35 and 

 50 per cent, alcohol to acid alcohol, leaving it 30 minutes in each. 



