170 FOOT-NOTES TO EVOLUTION. 
certain stages preliminary to their union, which are 
essentially alike. The animal egg is a large, more or 
less spherical cell, enveloped usually by 
certain membranes, containing a large 
nucleus and cytoplasm. The vast bulk of the egg 
cell, however, is made up of inert food material in the 
form of yolk granules, which are stored up in it as 
nourishment for the developing embryo. The nucleus, 
or germinal vesicle, is large, and contains a network of 
chromatin together with one or more conspicuous nucle- 
oli. There are three periods usually recognised in the 
development of the egg cell, viz.: 1. The period of 
multiplication; 2, the period of growth; and, 3, the 
period of maturation. The first period is characterized 
by a continued series of divisions of the primitive repro- 
ductive cell and its descendants, which produces a large 
number of “ovogonia.”’ Succeeding this is a period of 
growth in which the ovogonia increase greatly in size, 
mainly through the production and storing up of food 
yolk. At the close of this period the germ cell, now 
termed a “primary ovocyte,” enters upon the matura- 
tion period, in which it undergoes two divisions in rapid 
succession, by means of which two minute 
cells, the polar bodies, are cut off from 
the egg. Through these two divisions the number of 
chromosomes in the egg nucleus is reduced to one half 
that which is found in the other cells of the body. The 
first polar body also usually divides, and thus, at the 
close of the period of maturation, four cells result, one 
large mature egg cell, ready for the fertilization which 
initiates the development of the embryo, and three 
minute polar bodies, which are to be regarded simply as 
rudimentary eggs. The nuclei of these four cells are 
exactly alike in that they all contain the same number 
of chromosomes—i. e., one half the number in the somatic 
The egg cell. 
Maturation. 
