138 OUTLINES OF EVOLUTIONAKY BIOLOGY 



intelligible by a careful study of Fig. 68, which represents in a 

 very diagrammatic manner the formation of the polar bodies and 

 the distribution of the maternal and paternal chromosomes in the 

 maturation of a typical animal ovum. 



Periodic reduction of the number of chromosomes is clearly a 

 necessary consequence of the sexual process, for a doubling of 



FIG. 68. Diagram of the Maturation of a typical Animal Ovum, showing 

 the Behaviour of the Maternal and Paternal Chromosomes and the 

 Formation of Polar Bodies. (The somatic number of chromosomes is 

 supposed to be four ; the maternal chromosomes are shaded and the 

 paternal not, and the differences between the two of each set are 

 indicated by their shapes.) 



A, ordinary somatic mitosis in an oogonium, each chromosome split ; B, daughter 

 oogonium ; C, synapsis in primary oocyte ; D, reducing division, formation of first 

 polar body ; E, commencement of formation of second (or third) polar body by ordinary 

 mitosis and of division of the first ; F, mature ovum with three polar bodies. 



chr., chromosomes; c.s., centrosome ; n.m., nuclear membrane; p.b. 1 3, polar bodies. 



the number at every zygosis or conjugation of gametes could not 

 go on indefinitely without some such compensation. In animals, 

 as we have seen, the reduction takes place during the maturation 

 of the germ cells, but it is by no means necessarily associated with 

 this process. In the ferns, where alternating sexual and asexual 

 generations are represented by independent and well developed 

 organisms, the reduction takes place in the process of spore- 

 formation by the sporophyte. Hence the gametophyte (pro- 

 thallus) to which the spore gives rise has in all its somatic cells 



