MATURATION OF OVUM. 61 
atime in the precincts of the ovum, and may even divide. 
The second division follows the first without the intey- 
vention of the “resting stage” which usually succeeds a 
nuclear division. In most cases the division which forms 
the first polar body is a reducing or meiotic division, the 
number of chromosomes being reduced to half the number 
characteristic of the cells of the body. The extrusion of 
polar globules and the associated reduction is almost 
universal in the history of ova, but in most parthenogenetic 
ova only one polar body is formed, and there is no reduc- 
tion in the number of chromosomes. In some other cases 
B 
Fic. 29.—Diagram of maturation and fertilisation. 
(From Zvolution of Sex.) 
A. .Primitive sex cell, supposed to be ameeboid. 
B. Ovum; C. formation of first polar body (1. 4.2.); D. formation 
of second polar body (2. 4.4.). 
B’, Mother sperm cell; C’. the same divided (sperm-morula). 
2’. Ball of immature spermatozoa: sA., liberated spermatozoa. __ 
£. Process of fertilisation; #. approach of male and female nuclei 
within the ovum. 
the parthenogenetic ovum passes through the meiotic 
phase and forms two polar bodies. The second of these, 
however, is not liberated, but remains within the ovum and 
re-uniting with the reduced nucleus restores the normal 
number of chromosomes. 
Reducing or Meiotic Division.—In each kind of animal 
there is a definite number of chromosomes, say , in each 
of the body-cells. In the ripe germ-cells, however, there is 
half the normal number, 2, so that when spermatozoon and 
ovum unite in fertilisation the normal number is restored. 
In the history of the germ-cells, therefore, in one way or 
another, at one stage or another, the number of chromo- 
