CHAPTER IV. 

 FERTILIZATION. 



Mitosis, as described in Chapter I, is the process by which cells proliferate 

 to form the various tissues and organs and to take the place of cells worn out 

 in the carrying on of their labors or destroyed by injury. It is constantly going 

 on throughout the life of the individual. Attention has been called to the fact 

 that in all such reproduction there is a constantly maintained somatic number of 

 chromosomes. This has been seen to hold true up to the formation of the 

 mature germ cells — the mature ovum and the spermatozoon, each of which 

 contains one-half the somatic chromosome number. In all sexual reproduction 

 the starting point of the new individual, that is, the formation of the single cell 

 from which all the tissues and organs develop, is the union of two mature germ 

 cells, the spermatozoon and the ovum, one from the male the other from the 

 female. This union is known as fertilization and the resulting cell is the 

 fertilized ovum. 



The details of the process vary in different animals. Its essence is the 

 entrance of the spermatozoon into the ovum and the union of the nucleus of the 

 spermatozoon with the nucleus of the ovum. At the time of its entrance into 

 the egg, the sperm head is small and its chromatin extremely condensed 

 (Fig. 22, 2). Soon after entering the ovum, however, the sperm head un- 

 dergoes development into a typical nucleus, the male pronucleus (Figs. 22, 3, 

 and 13, C). This male pronucleus is to all appearances exactly similar in 

 structure to the nucleus of the egg, which latter is now known as the female 

 pronucleus. The chromatin networks in both pronuclei next pass into the 

 spireme stage, the spiremes segmenting into chromosomes of which each pro- 

 nucleus contains one-half the somatic number. The nuclear membranes mean- 

 while disappear and the chromosomes lie free in the cytoplasm. During these 

 changes in the pronuclei, the amphiaster has formed and the male and the 

 female chromosomes mingle in its equatorial plane (Fig. 22, 5). At this stage 

 no actual differentiation can be made between male chromosomes and female 

 chromosomes, the differentiation shown in Fig. 2j2, 5, being schematic. 

 The picture is now that of the end of the prophase of ordinary mitosis, 

 the somatic number of chromosomes being arranged in a plane midway 

 between the two centrosomes. With the mingling of male and female chro- 

 mosomes fertilization proper comes to an end. The further steps are also 

 identical with those of ordinary mitosis. Each chromosome splits longitudinally 



35 



