MALE AND FEMALE PRONUCLEI 



131 



genesis according to whether ova or spermatozoa are produced 

 thereby, is accompanied by nuclear phenomena of very great 

 interest, whereby the maturation of the germ cells is effected. 

 Before describing this process we must lay stress upon certain 

 preliminary considerations. 



As we have already seen, each kind of animal or plant is 

 characterized by the appearance of a definite number of chromo- 

 somes in the nuclei of its cells at the time when these are under- 

 going division by mitosis. Although not absolutely constant in 

 all cases the number is usually the same for all the different 

 somatic cells of which the body 

 is composed. It is usually an 

 even number, and (with certain 

 exceptions) it remains the same 

 in successive generations of 

 individuals. 



It will also be remembered 

 that the zygote or fertilized 

 egg from which the individual 

 develops is formed by the con- 

 jugation of two gametes, ovum 

 and spermatozoon, and that in 

 this process the nuclei of the 

 gametes, sometimes called the 

 male and female pronuclei, 

 unite, or at any rate co-operate 

 as a single nucleus. Fig. 64 is 

 taken from an actual photograph of an egg of Ascaris in process 

 of fertilization ; the spermatozoon has already entered the 

 ovum and the male and female pronuclei (pro.) are seen lying 

 side by side in the cytoplasm. Each pronucleus brings with it 

 its own set of chromosomes, and hence the zygote nucleus has 

 double the number of chromosomes possessed by either of the 

 gametes. Thus it appears at first sight that every conjugation 

 or sexual union of gametes (zygosis) must be accompanied 

 by a doubling of the number of chromosomes, and we might 

 therefore expect to find each successive generation with twice as 

 many chromosomes in its nuclei as the preceding one. That 

 this is not actually so depends (in the case of animals) upon the 

 fact that the nuclei of the gametes contain only half the number 

 of chromosomes characteristic of the somatic cells; if the 



K 2 



FIG. 64. Ovum of the Horse Worm 

 (Asmris megolocephala] during 

 the Process of Fertilization, 

 showing the male and female 

 Pronuclei (pro), X 770. (From 

 a photograph.) 



