I70 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 

 e egg ce . 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.: i. 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 divisipns 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 

 Maturation. ^^^^^^ ^^^ p^^^^. ^^^-^^^^ ^^^ ^^^ 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 



