FERTILIZATION 319 



called conjugation; in those instances where there is a great 

 difference in size, it is called fertilization. The conjugating 

 elements are called gametes. In case of fertilization the large 

 cell is called a macrogamete or egg, while the small one is called 

 a microgamete or sperm. Further, the individual giving origin 

 to a macrogamete is called female, and the one from which the 

 microgamete springs is a male. A further comparison between 

 male and female will be made a little later. 



695. Maturation. — In every case of conjugation or fertili- 

 zation the nuclei of the two cells sooner or later unite, and the 

 real significance of the process centres in the nuclei. The state- 

 ment has already been made that the number of chromosomes 

 is fixed for a given species, a condition that is maintained by the 

 splitting of the chromosomes at each cell division. On the 

 fusion of the nuclei, however, the number would be doubled 

 were it not for the prehminary process by which both nuclei 

 are prepared for the approaching'^ usion. To explain this proc- 

 ess we will take as an example the sperm cells of Ascaris megalo- 

 cephala. The typical number of chromosomes in this species 

 is four, and this is also the number in the cell divisions which 

 lead up to the formation of the sperm mother cells. The latter 

 remain for an unusually long period in the growing, resting 

 condition and attain unusually large size. When mitosis begins 

 it is seen that the four chromosomes have already spHt into 

 eight which, however, still remain paired. The four pairs ar- 

 arnge themselves in two groups of four (tetrads) in the equator of 

 the spindle, and the cell divides into two halves, after the usual 

 method. Then the centrosomes immediately divide and form 

 new spindles, so that a second division occurs before the nucleus 

 has entered the resting condition. In this second division the 

 four daughter chromosomes do not split, but one chromosome 

 from each of the original two tetrads moves toward each pole of 

 the spindle. There are thus formed four similar cells, each 

 with two chromosomes. These are the sperm cells. 



