Example 3 



Preparation of a Wholemount of a 48-hour Chick Embryo, 

 Using the Alum Hematoxylin Stain of Carazzi 



Fertile eggs are relatively easy to secure and should be incubated at a tem- 

 perature of 103° F. for the required period of time. The term "48-hour chick" 

 is relatively meaningless since the exact stage and development which will 

 have been reached after two days in the incubator depends on the temperature 

 of the latter, on the temperature at which the egg was stored prior to its incu- 

 bation, and even on the age of the hen. It is desirable therefore, if any very spe- 

 cific age of development is required, to start a series of eggs in the incubator at 

 three- or four-hour intervals and then to fix and mount these at the same time. 



For the removal of the embryos from the egg, one needs first a series of 

 finger bowls or any circular glass dishes of 5 to 6 in. in diameter and 2 to 3 

 in. in depth, a number of Syracuse watch glasses, a large quantity of a 0.9 per 

 cent solution of sodium chloride, a pair of large dissecting scissors, fairly fine 

 forceps, a pipette of the eye-dropper type, some coarse filter paper, and a pencil. 



No very great accuracy is required in the making up of the normal salt 

 solution, and any percentage between 0.7 and 1 will be sufllicient for the pur- 

 pose in mind. Though it is customarily specified that the temperature of the 

 solution should be 102 to 103° F., anywhere within 10° on either side of these 

 figures is relatively safe. The egg is removed from the incubator and placed in 

 one of the finger bowls, which is filled with the warm normal saline solution 

 until the egg is totally immersed. If the operator is rather skilled, it is possible, 

 of course, just to break the egg into the warm saline solution as though one 

 were breaking it into a frying pan, but it is recommended that the inexperi- 

 enced prepare several hundred wholemounts before they endeavor to do this. 

 The method by which the inexperienced can be assured of securing a perfect 

 embryo on every occasion is first to crack open the air space which lies at the 

 large end of the egg and to let the air bubble out through the warm saline 

 solution. This permits the yolk to fall down out of contact with the upper 

 surface of the shell, which may be removed with blunt-nosed forceps, working 

 from the air space toward the center. Again the matter of practice is involved, 

 for a skilled operator can remove this shell in large portions, while the inex- 

 perienced should work very carefully to avoid puncturing the yolk. If the yolk 

 is punctured, it is much simpler to throw the egg away and start with another 

 one. After about half the shell has been removed, it will be found quite easy 

 to tip the yolk, with the embryo lying on top of it, out into the saline solu- 

 tion and remove the shell. 



89 



