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 



B 



FIG. 11. DIAGRAM SHOWING THE FERTILIZATION OF THE OVUM 



A, the mature ovum with a central nucleus, or, as it is termed, female pro- 

 nucleus (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 ma- 

 ternal 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 pronucleus; f, cell wall; g, body of cell. 

 Redrawn from Cunningham, Anatomy. 



