56 ANIMAL PHYSIOLOGY. 
the most complicated known, inasmuch as it is adapted for. 
development outside of the body of the mother, and must, con- 
sequently, be capable of preserving its form and essential vital 
properties in a medium in which it is liable to undergo loss of 
water, protected as it now is with shell, etc., but which, at the 
Fic. 60.—Diagrammatic section of an unimpregnated fowl’s egg (Foster and Balfour, after 
Allen Thomson). bl, blastoderm or cicatricula ; w. y, white yolk ; y. y, yellow yelk ; ch. 1, 
chalaza ; i. s.m, inner layer of shell membrane ; s. m, outer layer of shell membrane; s, 
shell; a.c. h, air-space ; w, the white of the egg; v. t, vitelline membrane ; x, the denser 
albuminous layer lying next the vitelline membrane. 
same time, permits the entrance of oxygen and moisture, and 
conducts heat, all being essential for the development of the 
germ within this large food-mass. The shell serves, evidently, 
chiefly for protection, since the eggs of serpents (snakes, turtles, 
etc.) are provided only with a very tough membranous cover- 
ing, this answering every purpose in eggs buried in sand or 
otherwise protected as theirs usually are. As the hen’s egg is 
that most readily studied and most familiar, it may be well to 
describe it in somewhat further detail, as illustrated in the 
above figure, from the examination of which it will be ap- 
parent that the yelk itself is made up of a white and yellow 
portion distributed in alternating zones, and composed of cells 
of different microscopical appearances. The clear albumen is 
structureless. 
The relative distribution, and the nature of the accessory or 
non-essential parts of the hen’s egg, will be understood when it 
is remembered that, after leaving its seat of origin, which will 
be presently described, the ovum passes along a tube (oviduct) 
