214 ZOOLOGY. 



^ 



nucleus possessing capacities of development not existing 

 in either of the pronuclei alone. This union of pronuclei 

 is the essence of fertilisation, and the resulting nucleus is 

 called the first segmentation-nucleus. Every nucleus in 

 the whole body of an adult rabbit or any other animal has 

 been produced by division from one original nucleus the 

 first segmentation-nucleus of a fertilised ovum, and the 

 division takes place in such a way that material derived 

 from both pronuclei is found in every resultant nucleus. 



The most important discovery of recent years in connec- 

 tion with this subject is that the number of chromosomes 

 for any particular species is constant, is the same in all 

 the somatic cells of that species whenever they divide, and 

 that the number in the gametes is half of the number in 

 the somatic cells. Thus in the process t of fertilisation 

 male chromosomes do not unite with the female, each with 

 each, so that the total number in the fertilised ovum or 

 zygote remains the same, but each gamete contributes half 

 the specific number, and thus the whole specific number is 

 restored in every generation. The chief result therefore 

 of the formation of polar bodies or maturation divisions 

 is the reduction of the number of chromosomes in the 

 ovum to half the number in the somatic cells. In some 

 cases it has been proved that the spermatozoon also con- 

 tains half the number of chromosomes found in the 

 somatic cells, and that this result is brought about by two 

 divisions corresponding to the maturation divisions of the 

 ovum. But in most cases the male cells are too small to 

 permit of the chromosomes being counted. 



In reference to this reduction in the number of chromo- 

 somes, the two divisions which lead to the formation of the 

 mature gametes are called reduction divisions. In the case 

 of the ovum the two divisions produce one ovum and three 

 polar cells, for the first polar cell divides into two after its 

 separation. These three polar cells are regarded as de- 

 generate ova, so that the two reduction divisions produce 

 four female gametes, of which only one is capable of con- 

 jugation and development. Similarly in the development 

 of the male gametes or spermatozoa it has been shown in 

 some cases that two reduction divisions take place, but the . 



