CHAP. XII The Life-History of Animals 193 



The spermatozoon, though physiologically the comple- 

 ment of the ovum, is not its morphological equivalent. 

 The precise equivalent of the ovum is a primitive male-cell 

 or mother-sperm-cell, which divides repeatedly and forms a 

 ball or clump of spermatozoa. This division is to be com- 

 pared with the division or segmentation of the ovum, which 

 we shall afterwards discuss. 



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. Quite unique is the case of one of 

 Sir John Lubbock's queen-ants, which, thirteen years after 

 the last sexual union with a male, laid eggs which 

 developed. 



6. Maturation of the Ovum. — Most ova before they are 

 fertilised are subject to a remarkable change, the precise 

 meaning of which is not certainly known. The nucleus of 

 the ovum moves to the surface and is halved twice in rapid 

 succession. Two minute cells or polar globules are thus 

 extruded, and come to nothing, -while the bulk of the 

 nucleus is obviously reduced by three-fourths. It may be 

 that the ovum is only behaving as other cells do at the 

 limit of growth, or that it is exhibiting in an ineffective sort 

 of way the power of independent division which all the re- 

 productive cells of very simple many-celled animals perhaps 

 possessed ; it may be that it is parting with some surplus 

 material which is inconsistent with or no longer necessary 

 to its welfare, and there are other theories. One fact, 

 however, seems well established, that parthenogenetic ova, 

 which are able to develop into embryos without being 

 fertilised, extrude only one polar globule, a fact which 

 suggests that the amount of nucleus thus retained some- 

 how makes up for the absence of a spermatozoon. 



7. 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 



O 



