EXPERIMENTAL CHICK EMBRYOLOGY 



1+27 



Remove the mounting ring from the 95^ alcohol, let It air-dry hriefly, dip it into 

 1+5°C. melted paraffin, and place it on the egg so that -"t encircles the shell opening. 

 With a small water-color paint "brush paint melted paraffin onto the outer margins of the 

 mounting ring so that it is thoroi^hly sealed to the shell and no air can pass beneath. 

 Do not allow any paraffin to get into the egg. 



Secure a circular coverslip, previously cleaned, and holding it with forceps pass it 

 through a gas flame, exposing "both sui-faces. The coverslip should Just fit the mounting 

 ring. While still warm, "bring the coverslip into position on the mounting ring and gently 

 press it into place. The paraffin adherent to the mounting ring should melt and fasten 

 the coverslip tightly to it. Paint a ring of paraffin on the outer edge of the coverslip, 

 further sealing it to the egg shell. The entire operation consists simply of providing a 

 sealed window in the place of a limited amount of egg shell. Eetum the egg to the incu- 

 bator. In the same position, for the first 2k hours. Thereafter the egg may be rotated 

 somewhat, but the embryo and its membranes should be kept away from the window. 



Two modifications of the above procedure have been practiced, "but neither is neces- 

 sary. One is to puncture the air space so that it will be deflated and the embryo will 

 be further depressed away from the upper shell membrane. If this is done, simply cover 

 the puncture with paraffin or scotch tape. The second modification consists of adding 

 egg-albumen from a second egg, to fill up the space between the embryo and the glass win- 

 dow. While albumen is bacteriolytic, this practice Is ill advised, because it tends to 

 add to the infection hazzard through handling, and it generally clouds up the window. 



Of course, a glass window is not the only useful type. Pliofilm, cellophane, and 

 even Scotch tape have been used. If there is apt to be considerable delay between the 

 preparation of the host, and the graft tissues, it i.s advisable to protect the embryo with 

 a temporary Scotch tape covering of the shell opening. Square and round coversllps may be 

 used without a mounting ring, but paraffin sealing is the more dlffj.cult. 



The embryo may be examined from time to time but it must be remembered that with each 

 removal of the shell cap there is opportunity for bacterial infection and Increased evapo- 

 ration. Embryos may be carried through to hatching. 



THE SHELL CAP METHOD : 



This is the method of Price and Fowler (I9I+0). It consists of using a shell from the 

 blunt end of one egg to cover the exposed (blunt) end of an egg from which the shell has 

 been removed. This cover may be removed at periodic Intervals, and the embryo observed 

 through a much larger aperture than in the case of the cover glass window. There is, of 

 course, added danger of Infection. Instead of the shell cap, a fitted glass cap, the 

 edges of which are fastened to the shell, can be used and need not be removed to observe 

 the chick development within. This type of shell cap has the one disadvantage of not 

 allowing free transfer of respiratory gases. 



Save the blunt half of egg shells from any unincubated eggs 

 should be left in position, and the shells 

 may be sterilized in alcohol, to which a 

 little iodine is added. The cut edges of 

 the shell caps may be made smooth and less 

 brittle by dipping them in melted paraf- 

 fin. 



Candle a fertilized egg to mark the 

 margin of the air space at the blunt end. 

 Place the fertilized egg in a Syracuse 

 dish with Permoplast base so that the 

 blunt end is uppermost. Clean off the 

 entire blunt end with iodized 95')^ alcohol, 

 and wipe it dry with sterile cotton. 

 With needle, lancet, or forceps make a 

 small hole in the center of the blunt end 

 and then gradually pick away the shell. 



The shell membrane 



