THE EGG 



19 



The 3'olk 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 volk, 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 

 concentric layers of white yolk concentric to the 

 inner bulb-shaped mass.^ If an egg be opened, 

 a delicate hair inserted in the blastoderm to 

 mark its position, and then boiled hard, a sec- 

 tion 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, quite different 

 in each (Fig. 3). Those of the yellow yolk are 

 on the whole larger than those of the white 

 yolk (about 0.025-0.100 mm. in diameter) with 

 extremely fine granular contents. There is no 

 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 yolk- 

 sphere. 



^ The assertion that the thin layers that define the concentric stratifica- 

 tion of the yellow yolk are of the nature of white yolk is traceable to Meckel 

 V. Hemsbach, Leuckart, and Allen Thomson. His was not able to satisfy 

 himself that the characteristic elements of the white yolk occur within these 

 thin concentric lamellae (Untersuchungen ueber die erste Anlage des Wir- 

 beltierleibes, p. 2). 



