CHAPTER X 



SEXUAL REPRODUCTION AND DEVELOPMENT OF 



THE INDIVIDUAL* 



Eeproduction is one of the fundamental capacities of all living- 

 things. New individuals of the same kind can be produced sexually 

 by the activity of special germ cells (gametes) provided for that 

 purpose in metazoan animals. The germ cells of the two sexes are 

 quite different. The processes involved in sexual reproduction and 

 embryology are generally similar in different groups of animals but 

 different in matters of detail. 



The male germ cells or spermatozoa are produced in the male 

 gonads or testes. The internal structure of more highly developed 

 testes consists largely of extensive seminiferous tubules imbedded in 

 interstitial tissue. The cells composing the inner lining layers of 

 these tubules produce the spermatozoa by a process of maturation. 

 The female germ cells or ofa are produced in the female gonad or 

 ovary. Certain cells of the inner surface of the epithelial covering 

 of the ovary become differentiated as germ cells. They become im- 

 bedded in the stroma of the ovary in the process of development. 

 The original epithelial cells from which germ cells arise are known 

 as primordial germ cells in either sex. 



The maturation (gametogenesis) or development of germ cells 

 consists of a series of cell divisions which is modified at one point 

 to bring about a fusion of chromosomes and subsequently a reduc- 

 tion in their number. In the case of development of spermatozoa 

 the maturation process is completed within the gonad, but in devel- 

 opment of ova the last division or in some cases last two divisions 

 are completed after leaving the gonad and uniting with a sperma- 

 tozoon. The maturation process prepares the germ cells for fertili- 

 zation. Development of the male germ cell is known as sperma- 

 togenesis, and the development of the female germ cells is oogenesis. 



Oogenesis begins with the primordial germ cell within the ovary. 

 These cells are typically spherical or oval with a prominent nucleus, 

 having the normal number of chromosomes for the somatic cells of 

 the species. This number of chromosomes is known as the diploid 

 number. For purposes of illustration the process will be described 



♦For a more complete study of comparative embrj^ology, the student is referred 

 to Richards, Outline of Comparative Embryology, published by John Wiley & Sons, 

 Inc., or to the chapter by Dr. Richards on comparative embryology in Potter, Text- 

 book of Zoology. 



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