THE NATURAL HISTORY OF MAN 41 



separate race. Additional interest is given to this classification 

 of the races as it corresponds in a general way to their geo- 

 graphic distribution as shown in Fig. 2. 



a b 



Fig. 11. Diagram showing the fertilization of the ovum. A, the mature 

 ovum with a central nucleus, or, as it is termed, female pronucleus (e), 

 containing the reduced number of chromosomes; the others have been given 

 off into the first and second polar bodies (a a'). A single sperm (b) is 

 entering the ovum. B, later stage in which the male pronucleus (d), with 

 the two paternal chromosomes, has formed from the sperm. The sperm 

 also brings to the ovum the centrosome (c) which is regarded as the dynamic 

 division center. C, still later stage in which the male and female pronuclei 

 are uniting to form, in stage D, the segmentation nucleus (e') which contains 

 an equal amount of maternal and paternal chromatin. Following this the 

 fertilized egg cell will divide by the complicated process of mitosis which 

 insures the equal division of the chromatin material among the daughter 

 cells, a, first polar body (divided into two parts) ; a', second polar body; 

 b, sperm entering the ovum; c, centrosome (later divides); d, male pro- 

 nucleus; f, cell wall; g, body of cell. Redrawn from Cunningham, Anatomy. 



