20 THE FIRST PRINCIPLES OF HEREDITY 



propagation of the species. We must, therefore, look 

 upon this as the main function of sexual reproduction also. 

 We may conveniently deal with sexual reproduction 

 under four heads : (a) Heterogamy ; (b) Autogamy ; (c) Par- 

 thenogenesis ; and (d) Alternation of Generations. 



(a) Heterogamy. 



Heterogamy means reproduction by union of differ- 

 entiated male and female individuals. This is, of course, 

 the best known, because made familiar to us by its occur- 



rence in all higher animals 

 and man. Here we have 

 the union of the separate 

 male and female organ- 

 isms, in order to produce 

 the new progeny. Among 

 the Protistes, as described 

 under Conjugation, the 

 fusion takes place bodily ; 

 but with Metazoa and 

 Metaphyta this would 

 evidently be impossible. 



FIG. i3.-VoLvox MINOR. After Now, we have seen already 

 Stein.) 



(After 



(From Weismann, " The Germ- 

 Plasm."} 



sZy body cells ; kz, germ cells. 



in the evolution of asexual 

 reproduction that distinct 

 parts of the body may be 

 set aside for the purpose 

 of propagation, as in 

 sporulation, where the spores may be looked upon as 

 asexual germ-cells. A similar process of differentiation 

 between body and propagating germ-cells takes place 

 among the Metazoa. We can see this illustrated beauti- 

 fully in the Volvox, which may be described as a cell- 

 colony of one-celled animals. Here some of the uniform 

 body-cells become distinct from the others, and develop into 

 sex-cells, these alone being able to reproduce the species. 



