EXPERIMENTAL CHICK EMBRYOLOGY 



No attempt will be made here to describe the most delicate of the transplantation 

 operations (e.g., those by Willier and by Hamburger and their students) that have been 

 performed on the chick embryo. The student is, however, directed to the work of these 

 investigators, particularly to inter-specific transplantations. 



Hecent work of Spratt (l9'+7, 19'+8) has demonstrated that the extreme precautions of 

 a dust-free operating room, ultra-violet lighting, masking, etc., are not necessary so 

 long as reasonable precautions are taken to avoid the actual introduction of bacteria in- 

 to the hen's egg. It is therefore safe to predict that the chick embryo will be used in- 

 creasingly in courses in Experimental Embryology, and that it will supplement the work on 

 the amphibia admirably well. The hen's egg is available during the months when amphibian 

 material is scarce. 



Aside from the general procedures for handling the hen's egg in the laboratory, the 

 experimental procedures to be described will include (1) Detenaination of morphogeneti c 

 movements by means of vital dyes and charcoal particles, (2) Explantations or the cultur- 

 Ing of Isolates on artificial media (5) Chorio-allantoic grafting and (ij-) Transplantations. 



This exercise has the single experimental object, the hen's egg. It involves four 

 different procedures and in this respect this exercise cannot be weighed with the others 

 of this Manual. It Is recommended that the material on the hen's egg be assigned to the 

 second half of the second (Spring) semester, when the amphibian material is no longer 

 available and after the student has had the benefit of some months of experimental and 

 operational experience with amphibian material. 



THE PROCURING AND CARE OF LIVING MATERIAL 



Procuring of eggs : One must establish a reliable source of highly fertile eggs, 

 which prove to be perfectly normal in development. There are the usual seasonal varia- 

 tions in fertility with the low point during late Summer and Pkll, and the high point Just 

 after the peak of the winter. With optimum conditions fertility may reach as high as 90^ 

 but the low point may go to 2jja or even less. There is no evidence that any particular 

 breed lends Itself better than others to operational procedures. However, certain flocks 

 of hens give more viable eggs, particiilarly when they are provided with adequate sunlight 



Storing of eggs : If possible, eggs should be used shortly after being layed. How- 

 ever, eggs may be stored at cool ten^eratures (10°C.) for as long as a week. The percen- 

 tage of development will drop rapidly thereafter. 



Incubators : The small (Oakes) incubators will hold several dozen eggs and are quite 

 satisfactory although they are not provided with as sensitive temperature and humidity 

 controls, and forced-air draft for ventilation, as are other and larger models. The large 

 (Buffalo) incubators are excellent for the incubation of larger numbers of eggs, and the 

 physical factors are well controlled. Place the Incubator away from drafts and sunlight. 



Temperature control : Most incubators are provided with temperature control devices 

 regulated close to the optimum range of 105° - 105°F. (about 57.5°C.). If the Incubator 

 is not provided with forced draft, the temperature should be regulated at about 100°F. 

 Since the heated air tends to rise, the hanging thermometer should have its mercury bulb 

 at the level of the eggs. Each inch above the eggs the temperature may register at least 

 a degree higher than at the egg level. Once controlled incubation has begun it should not 

 be interrupted if normal development is desired. 



Bimidity control : A relative humidity of 60^ is best, although higher humidity, is 

 not deleterious. In the smaller Incubators several finger bowls of water should be placed 

 among the eggs. In the larger incubators there are generally large pans beneath the eggs 

 which pans should be filled with sand and kept constantly moist. Dehydration is one of 

 the commonest contributors to lethality. 



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