CHAPTER IV. 

 FERTILIZATION. 



When the complex maturation processes described in the preceding chapter 

 are completed, the spermatozoon is ready for union with the mature ovum. 

 This union, which forms the starting point of a new individual in all sexual 

 reproduction, is known as fertilization, and the resulting cell is the fertilized 

 oi'iim. 



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 under- 

 goes development into a typical nucleus, the male pronudeus (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 pronucleus contains one- 

 half the somatic number. The nuclear membranes meanwhile disappear and 

 the chromosomes lie free in the cytoplasm. During these changes in the pro- 

 nuclei, the amphiaster has formed and the male and the female chromosomes 

 mingle in its equatorial plane (Fig. 22, 5). At this stage no actual differentia- 

 tion can be made between male chromosomes and female chromosomes, the 

 differentiation shown in Fig. 22, 5, being schematic. The picture is now that 

 of the end of the prophase of ordinary mitosis, the somatic number of chromo- 

 somes being arranged in a plane midway between the two centrosomes. With 

 the mingling of male and female chromosomes fertilization proper comes to an 

 end. The further steps are also identical with those of ordinary mitosis. Each 

 chromosome splits longitudinally into two exactly similar parts (Fig. 22, 5), 

 one of which is contributed to each daughter nucleus (Fig. 22, 6), and the cell 

 body divides into two equal parts. (For details of succeeding anaphase and 

 telophase see p. 6.) There thus result from the first division of the fertilized 

 ovum, two cells which are apparently exactly alike and each of which contains 

 exactly the same amount of male and of female chromosome elements (Fig. 22, 6). 



The amphiaster of the fertilized ovum appears to develop as in ordinary 

 mitosis. As to the origin of the centrosomes, however, much uncertainty still 



33 



