CHAPTER III. 

 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 ovum, or zygote. 



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. 

 Soon after entering the ovum, however, the sperm head undergoes develop- 

 ment into a typical nucleus, the male pronucleus. 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 chromo- 

 somes 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. At this stage no actual differentiation can be made 

 between male chromosomes and female chromosomes, the differentiation 

 shown in Fig. 15 being schematic. The picture is now that of the end of the 

 prophase of ordinary mitosis, the somatic number of chromsomes being 

 arranged in a plane midway between the two centrosomes. With the ming- 

 ling 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, one of which 

 is contributed to each daughter nucleus, and the cell body divides into two 

 equal parts. 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. 



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

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

 still exists. The middle piece of the spermatozoon always enters the ovum 

 with the head. It has already been shown that one or two spermatid centro- 



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