Archive image from page 36 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 The yolk and blastoderm are enclosed within the delicate vitelline membrane; the yolk is a highly nutritious food destined to be gradually digested and absorbed by the living cells of the blastoderm and used for the growth of the embryo. It is not of uniform composition throughout, but consists of two main ingredients known as the yellow and the white yolk. The yellow yolk makes up the greater part of the yolk-sphere; the main part of the
Archive image from page 36 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 The yolk and blastoderm are enclosed within the delicate vitelline membrane; the yolk is a highly nutritious food destined to be gradually digested and absorbed by the living cells of the blastoderm and used for the growth of the embryo. It is not of uniform composition throughout, but consists of two main ingredients known as the yellow and the white yolk. The yellow yolk makes up the greater part of the yolk-sphere; the main part of the white yolk is a flask-shaped mass, the bulb of which, known as the latebra, is situated near the center of the whole yolk, the neck rising towards the surface and expanding in the form of a disc (nucleus of Pander) situated imme- diately beneath the blastoderm (Fig. 2); at its margin this disc is continuous with a thin peri- pheral layer of white yolk that surrounds the entire mass. In addition there are several thin layers of white yolk concentric to the inner bulb- shaped mass. If an egg be opened, a dehcate hair inserted in the blastoderm to mark its po- sition, and then boiled hard, a section through the hair and center of the yolk will show the above relations quite clearly. The white yolk does not coagulate so readily as the yellow yolk, and it may be distinguished by this property as well as by its lighter color. Both kinds of yolk are made up of innumer- able spheres which are, however, c|uite different in each (Fig. 3). Those of the yellow yolk are on the whole larger than those of the white yolk (about mm. in diameter) with „. . extremely fine granular contents. There is no P®- fluid between the spheres. Those of the white yolk are smaller and more variable in size, ranging from the finest granules up to B Fig. 3. — Y o 1 k - spheres of the hen's egg; highly magnified. (After Foster and Bal- four.) A. Varieties of white yolk-spheres. B. Yellow yol
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