GENETIC THEORY 3 



determination could now be stated more exactly. In a wide 

 variety of species sex-dimorphism occurs; within them there 

 are males and females. Sexual reproduction, taking the form 

 of the fusion, permanently or temporarily, of two complete 

 individuals or parts thereof or in the union of single cells 

 derived from one and the same individual or from two 

 individuals, occurs in all those groups of organisms in 

 which the presence of a nucleus has been demonstrated. 

 In the higher forms sexual reproduction consists in the 

 formation of single cells, the gametes, the separation of 

 these from the rest of the individual and their subsequent 

 fusion in pairs to form the zygotes, the new individuals of 

 a new generation. In most of these forms two kinds of 

 gametes are to be found — a small, active, motile gamete 

 elaborated by the male (or by the testis of the herma- 

 phrodite) and a relatively large, inactive, non-motile gamete 

 elaborated by the female (or by the ovary of the herma- 

 phrodite). These gametes constitute the only organic bridge 

 connecting the generations. 



Usually during spermatogenesis the nucleus and the 

 cytoplasm of a cell are equally divided among four functional 

 spermatozoa, whilst during oogenesis three of the products 

 of division are suppressed and extruded to become the polar 

 bodies, only one remaining to become the functional ovum. 



The simplest form of spermatozoon consists of four parts: 

 (i) the nucleus forming the head; (2) the centrosome, a non- 

 nuclear body forming the middle piece from which the 

 axial filament of the vibratile tail is developed; (3) the mito- 

 chondria, bodies of non-nuclear origin which form the 

 sheath of this filament, and (4) ordinary cytoplasm which 

 forms a thin coat over the head and tail. The nucleus alone 

 is the constant constituent of the spermatozoon; it alone 

 fuses with the nucleus of the ovum. No other organ save 

 the centrosome takes any part in the development of the 

 new individual. 



The essential feature of sexuality is the production of two 

 different kinds of gametes by the individuals of a species, 

 male-type by the male, female-type by the female. Fusion 

 between gametes is restricted to such as possess and display 



