SECTION IX. 

 THE PHYSIOLOGY OF REPRODUCTION. 



With the exception of the phenomenon of consciousness, no 

 fact of life excites more interest and seems to offer greater diffi- 

 culties to an adequate explanation than the function of reproduc- 

 tion. The male cell (spermatozoon) and the female cell (ovum) 

 unite to form a new cell which thereupon begins to grow rapidly 

 and produces an organism that in all of its manifold peculiarities 

 of structure and function is essentially a replica of its parents. 

 The fundamental problems presented in this act of reproduction 

 are those of fertilization and heredity. In the former we must 

 ascertain why the union of the two cells is necessary or advanta- 

 geous, and the secret of the stimulating influence upon growth that 

 arises from this union. Under the term heredity we express the obvi- 

 ous, yet mysterious, fact that the fertilized ovum of each species de- 

 velops into a structure like that of its parents. Both of these im- 

 portant problems are essentially of a physiological character, that 

 is, they deal with properties of the living material composing the 

 reproductive cells; but, at present, biological investigation along 

 these lines is largely in the morphological stage. The part of the sub- 

 ject that can be studied with most success is the structural changes 

 that are associated with fertilization and reproduction. Great, 

 indeed wonderful, progress has been made during the last century, 

 but it is needless perhaps to say that much remains unexplained, 

 and- that in this, as in so many other problems of nature, the greater 

 our knowledge the clearer becomes our vision of the difficulties and 

 complexities of a final scientific explanation. Outside these funda- 

 mental problems there are other accessoiy functions connected, for 

 instance, with the external genital organs which in a measure are of 

 more immediate practical interest. In one way or another these 

 functions are necessary or helpful to the final union of the repro- 

 ductive cells. They form a part of the reproductive life which comes 

 more immediately under our observation and control, and consti- 

 tute, therefore, a subject which has been more accessible to in- 

 vestigation. In the brief treatment given in the following chap- 

 ters more emphasis is laid upon this side, the accessory phenomena 

 of reproduction, than upon the deeper, more fundamental prob- 



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