METAZOAN ORGANIZATION 1]5 



cells in each case. This large cell is called the secondary oocyte and 

 the small one is the first polar body. Each of these cells has four 

 chromosomes. Following this the secondary oocyte divides to form 

 the mature ovum and another polar body. Occasionally the first 

 polar body divides, but none of them have any further significance 

 after cariying away half of the chromosomes. They now degenerate, 

 and their protoplasm is reabsorbed by the surrounding tissue. The 

 series of divisions and changes following the primary oocyte stage 

 constitute the maturatio7i period of the process. The ovum contain- 

 ing the haploid number of chromosomes is now prepared to unite 

 with a mature spermatozoon in fertilization. 



Spermatogenesis is completed within the tubules of the testis, and, 

 like oogenesis, is a series of mitotic cell divisions. The primordial 

 germ cells divide by mitosis to form spermatogonia, and this process 

 continues just as it does in oogenesis, until the division of the pri- 

 mary spermatocytes which have developed during the growth period. 

 When the primary spermatocytes divide, the division is an equal one 

 and all of the resulting cells are typical secondary spermatocytes 

 with the haploid number of chromosomes. These cells divide to form 

 spermatids. Each spermatid then undergoes a change of shape or 

 transformation to form the mature spermatozoa, each with its half 

 number or, in this ease, four chromosomes. The change from sper- 

 matid to spermatozoa does not involve a cell division but simply rear- 

 rangement. The spermatozoon is a slender, motile cell composed of 

 head, middle piece, and tail. It is now able to swim in fluid and 

 prepared to unite with a mature ovum. 



The maturation process is very significant for at least two impor- 

 tant reasons. First, during the fusion and subsequent divisions of 

 the cells, there is given opportunity for variation of the genetic com- 

 position. Secondly, the number of chromosomes is reduced to half 

 in each mature germ cell, thereby making it possible for the germ 

 cells to unite without doubling the typical number of chromosomes 

 in each new generation. Each species has a definite and constant 

 number of chromosomes. 



Fertilization involves the union of a mature ovum and mature 

 spermatozoon to produce a fertilized ovum or zygote. The sper- 

 matozoon SAvims to the q^q and enters it by penetrating the outer 

 membrane which is called the vitelline membrane. For most animals, 

 as soon as one sperm enters an egg, the chemical nature of the vitelline 



