15 • THE DEVELOPMENT OF THE 

 CHICK EMBRYO 



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ithout doubt our knowledge regarding the details of structure and 

 function of the parts of the developing chick embryo is more extensive than 

 for any other species. And yet even in the chick the information about 

 cleavage and gastrulation leaves much to be desired. In part this situation 

 arises from the fact that these processes take place in the oviduct of the hen 

 before the egg is laid. Eggs from the oviduct are relatively inaccessible, and 

 the large numbers of them required for experimental studies are difficult to 

 obtain. The ovarian egg of the hen is large by virtue of the deposition of 

 enough yolk to sustain development for about three weeks at 103° F. The 

 yolk is composed of discrete particles, and a chemical analysis reveals a 

 phosphorus-containing protein, a globulin-like protein, and a protein to 

 which a carbohydrate is attached. Fats are well represented in the form of 

 lecithin, sterols, and neutral fats. Vitamins A, Bj, B 2 , niacin, D, and E are 

 present in yolk. The combined albumen and yolk contains the following 

 chemical elements: sodium, potassium, calcium, magnesium, iron, phos- 

 phorus, chlorine, iodine, bromine, sulphur, nitrogen, carbon, hydrogen, 

 oxygen, copper, barium, aluminum, manganese, strontium, titanium, vana- 

 dium, silicon, selenium, molybdenum, ruthenium, and boron. Other elements 



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