THE MULTIPLICATION OF ANIMALS AND SEX 57 



33. Sex, or male and female. In the slightly complex 

 animals one individual produces both egg cells and sperm 

 cells. But in the Siphonophora, or colonial jelly-fishes, stud- 

 ied in the last chapter, certain members of the colony pro- 

 duce only sperm cells, and certain other members of the 

 colony produce only egg cells. If the Siphonophora be 

 considered an individual organism and not a colony com- 

 posed of many individuals, then, of course, it is like the 

 others of the slightly complex animals in this respect. But 

 as soon as we rise higher in the scale of animal life, as soon 

 as we study the more complex animals, we find that the 

 egg cells and sperm cells are almost always produced by 

 different individuals. Those individuals which produce 

 egg cells are called female, and those which produce sperm 

 cells are called male. There are two sexes. Male and 

 female are terms usually applied only to individuals, but 

 it is evidently fair to call the egg cells the female reproduc- 

 tive cells, and the sperm cells the male reproductive cells. 

 A single individual of the simpler kinds of animals pro- 

 duces both male and female cells. But such an individual 

 can not be said to be either male or female ; it is sexless 

 that is, sex is something which appears only after a certain 

 degree of structural and physiological differentiation is 

 reached. It is true that even among many of the higher 

 or complex animals certain species are not represented by 

 male and female individuals, any individual of the species 

 being able to produce both male and female cells. But this 

 is the exception. 



34. The object of sex. Among almost all the complex 

 animals it is necessary that there be a conjugation of male 

 and female reproductive cells in order that a new individual 

 may be produced. This necessity first appears, we remem- 

 ber, among very simple animals. This intermixing of body 

 substance from two distinct individuals, and the develop- 

 ment therefrom of the new individual, is a phenomenon 

 which takes place through the whole scale of animal life. 



