THE EGG 19 
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 dise (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 A 
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 
vellow yolk, and it may be distinguished by this 
B 
Higseoy— Yuowl kk 
: ; spheres of the 
property as well as by its lighter color. Hens cee highly 
Both kinds of yolk are made up of innumer- magnified. (After 
able spheres which are, however, quite different Foster and Bal- 
in each (Fig. 3). Those of the yellow yolk are four.) 
on the whole larger than those of the white ee eee 
yolk (about 0.025—-0.100 mm. in diameter) with B. Yellow yolk- 
extremely fine granular contents. There is no “Phere: 
fluid between the spheres. Those of the white yolk are smaller 
and more variable in size, ranging from the finest granules up to 
1 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). 
