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 only increases as the square, of the radius. 

 Thus the proportion of surface to mass diminishes with increased 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 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 



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