CHAPTER XXI 

 THE PHYSIOLOGY OF REPRODUCTION 



SECTION I 

 THE ESSENTIAL FEATURES OF THE SEXUAL PROCESS 



THE two fundamental characteristics of protoplasm, which distinguish it 

 above all others from unorganised matter, are growth and activity. Growth 

 occurs at the expense of surrounding non-living material, while activity is 

 in every case an adapted reaction to changes in the environment. The 

 second characteristic would seem to involve a limitation of the first, and 

 does in fact determine the conditions under which it may occur. In the 

 process of growth of a minute spherical mass of protoplasm, its bulk and 

 mass increase as the cube, while the surface increases only as the square, of. 

 the radius. Thus the proportion of surface to mass diminishes with in- 

 creased size of the protoplasmic unit and, since activity is a function of the 

 surface, the larger the unit the smaller must be its activity. It follows that 

 there must be a limiting size to the living protoplasmic unit, and it is on 

 this account that practically no unicellular animal or plant exceeds a fraction 

 of a millimetre in diameter. If an organism is to attain any larger size, this 

 can only be by a multiplication of units, each presenting the same relative 

 amount of surface as a complete unicellular organism, though the surface 

 may be exposed to an internal and not to an external medium. Another 

 factor, limiting the size of the unicellular organism or of the unit of the 

 multicellular organism, is the necessity for maintaining a certain proportion 

 between the size of the nucleus and that of the cytoplasm composing the 

 body of the cell. Observations on artificial division of cells have shown 

 us that the functions of digestion, assimilation, and growth depend upon the 

 presence of a nucleus. Hence, when for any reason it is advantageous that 

 a cell should attain a large size, such a cell is almost always found to contain 

 many nuclei. All the ' giant cells ' found in the body of man under normal 

 or pathological conditions are also multinuclear. 



Thus the continuous display of the functions of assimilation and dis- 

 similation, of growth and activity, is possible only so long as cell division 

 keeps pace with growth. In unicellular organisms, under favourable con- 

 ditions, this growth and multiplication occur with prodigious rapidity. It 

 has been computed that a paramcecium, freely supplied with food material, 

 would, by growth and division, in the course of a year represent a mass of 

 protoplasm the size of the earth, assuming of course that no accidents or 



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