26 HEREDITARY CHARACTERS 



fertilisation have been made, particularly during recent years. 

 We have already seen that for the production of a new 

 multicellular individual, two cells derived from two indi- 

 viduals of the same species generally fuse, forming a single 

 cell the fertilised ovum. In both animals and plants, the 

 cells that fuse to form the fertilised ovum are the cell 

 produced by the male parent the sperm, and the cell 

 produced by the female parent the ovum. We have 

 already considered the phenomena which bring about the 

 reduction in the number of chromosomes in the reproductive 

 cells. This process is identical in both the sperms and the 

 ova. In other respects, however, there are some differences 

 between the processes undergone by these two kinds of 

 reproductive cells before they are ready to fuse and form the 

 fertilised ovum. In the male a certain group of cells is 

 differentiated at some period during the life of the organism, 

 and is destined to produce sperms. These cells contain the 

 full number of chromosomes, and they multiply throughout 

 the active sexual life of the organism. The process of 

 reduction begins to occur when the organism approaches 

 maturity, and thenceforward takes place continuously, those 

 cells containing half the number of chromosomes being 

 sooner or later separated from the body. All the reduced 

 cells form sperms, and all of these sperms are capable of 

 fertilising an ovum. Generally a vastly greater number 

 of sperms are produced than can possibly be utilised in 

 fertilising ova, but nevertheless all of them are capable of 

 doing so. 



The case of the ova is somewhat different. In the higher 

 animals they are differentiated at a very early period in the 

 life of the organism. In plants and in some animals they 

 may be differentiated at varying times, sometimes quite late 

 in the life of the organism. In every case, however, the ova 

 at first contain the full number of chromosomes. Before the 

 ovum is ready for fertilisation its nucleus divides meiotically 

 and reduction takes place. One of these nuclei remains 

 in the bulk of the cytoplasm of the ovum ; the other is 



