SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 211 



cance after carrying 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 maturation period of the process. The 

 ovum containing the haploid number of chromosomes is now pre- 

 pared to unite with a mature spermatozoon in fertilization. 



Sper7natoge7iesis is completed within the tubules of the testis, and, 

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

 cells divide by mitosis to form spermatogonia, and this process con- 

 tinues just as it does in oogenesis, until the division of the primary 

 spermatocytes which have developed during the growth period. 

 When the primary spermatocytes divide, the division is an equal 

 meiotic 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 spermatozoon with 

 its half number or, in this case, four chromosomes. The change 

 from spermatid to spermatozoon does not involve a cell division but 

 simply rearrangement. 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. These modified divisions of the oocytes in 

 oogenesis and spermatocytes in spermatogenesis, w^hich bring about 

 the reduction of chromosomes, are termed meiosis. 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 swims to the egg 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 

 membrane changes and prevents entrance of others. The head of the 

 sperm carries the nucleus and soon takes the form of a rounded male 

 pronucleus inside the cytoplasm of the egg. The egg nucleus is known 

 as the female pronucleus. The male and female pronuclei finallj^ fuse 



