310 THE STUDY OF ANIMAL LIFE CHAP. 



(oogonia), divide and redivide, often forming balls of 

 cells. They give rise to spermatocytes, which also 

 multiply by division. The last chapter in the sperma- 

 togenesis is the transformation of spermatocytes into 

 spermatozoa. 



In some cases spermatozoa which have been transferred 

 to a female may lie long dormant there. Thus those 

 received by the queen-bee during her nuptial flight may 

 last for a whole season, or even for three seasons, during 

 which they are used in fertilising those ova which develop 

 into workers or queen-bees. 



7. Maturation of the Ovum. Before an egg-cell is 

 fertilised it usually exhibits a remarkable process of 

 maturation. The nucleus moves to the surface and 

 divides twice in rapid succession, forming two minute 

 cells or polar bodies, which are extruded and come to 

 nothing. The first of these two divisions is usually quite 

 unique, for by it the number of chromosomes is reduced 

 to half the normal number, whereas in ordinary nuclear 

 division each chromosome is cleft longitudinally and the 

 number remains the same. In the formation of the sper- 

 matozoa there is a similar reducing or meiotic division, 

 and thus when the spermatozoon and ovum unite in 

 fertilisation, the normal number of chromosomes is 

 restored. The reduction of the chromosomes by a half 

 is probably of great importance in connection with 

 variation and hereditv. It enables us to understand 



/ 



how certain items in the inheritance may drop out alto- 

 gether, or how they may be represented only in a certain 

 proportion of the offspring. 



8. Fertilisation. When a pollen grain is carried by an 

 insect or by the wind to the stigma of a flower, it grows 

 down through the tissue of the pistil until it reaches the 

 ovule and the egg-cell which that contains. Then a 

 nuclear element belonging to the pollen cell unites with 

 the nucleus of the egg-cell. The union is intimate and 

 complete. 



When spermatozoa come in contact with the egg-shell 

 of a cockroach ovum, they move round and round it in 

 varying orbits until one finds entrance through a minute 



