102 A PRIMER OF BIOLOGY 



made for the continuance of the race, and this is 

 attained in one or both of two ways, i.e., by separation 

 of a part of the body of the parent capable of giving 

 rise directly to a new organism of the same type, 

 in other words, by vegetative and " asexual reproduc- 

 tion," or by separation of a cell an ovum or egg- 

 cell which is itself, save in exceptional cases, in- 

 capable of developing into a new organism without 

 previous fusion with a corresponding cell a, sperm 

 or fertilising cell almost always in animals, and very 

 generally in plants, derived from another individual. 

 This latter method is termed " sexual reproduction." 

 It is customary to speak of the sperm- producing 

 parent as the male, and the ovum-producing parent 

 as the female. The ovum, after fusion with the sperm, 

 becomes the oosperm and develops into the embryo 

 and, finally, into the adult. 



Let us inquire first as to the theoretical origin of 

 these two kinds of cells. 



One of the first things we become acquainted with 

 when we study the origin of cells is that they are 

 capable of division. Why does a cell divide ? A 

 cell is a living unit ; in it, during life, certain con- 

 structive changes are going on, tending to the accumu- 

 lation of organic substances and of energy in the 

 potential form, and, at the same time, certain destruc- 

 tive changes, tending to the liberation of potential 

 energy in the kinetic form, the decomposition of 

 complex compounds, and the formation of simpler 

 degradation products or excreta. We have seen 

 already that the surface of the cell is the medium 

 through which all nutritive substances must enter 

 the cell, and it is equally obvious that from that 

 same surface the waste products must be given off. 

 If, in consequence of adequate nutrition, the cell 



