CHAPTER XIII. 

 REPRODUCTION. 



VERY many facts in our knowledge of reproduction depend 

 on observations made upon lower animals, but there is sufficient 

 analogy between the known facts connected with human repro- 

 duction and development and those of the same stages in other 

 groups of beings to enable us to present, as at least approxi- 

 mately accurate, certain broad principles regarding the process 

 as it pertains to the human race. 



In order that a human being may be brought into existence it 

 is necessary that there be a union of the male element, the sper- 

 matozoon, and the female element, the ovum. Both these sexual 

 cells are developed from epithelium the spermatozoon from 

 that of the seminiferous tubules of the male, and the ovum from 

 the germinal layer of the ovary. 



In what follows reference will be had to reproductive proc- 

 esses in the human being. 



Spermatozoa. Human spermatozoa (Fig. 92) are elongated 

 bodies, about one five-hundredth of an inch in length, and con- 

 sist of three parts, head, mid-portion and tail. The last-named 

 part is about four-fifths the length of the entire spermatozoon. 

 The head is egg-shaped and much the thickest part of the ele : 

 ment. A slender filament, the axial fiber, extends throughout 

 its length from head to tail and projects slightly beyond the latter. 

 Spermatozoa are possessed of wonderful vitality. They live for 

 several weeks in the genital passages of the female. In the male 

 genital passages they may live for months in a quiescent state. 

 The nucleus is the fertilizing agent. Spermatozoa are also 



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