PARTHENOGENESIS. 105 



In reality sexual reproduction is nothing else than a special form 

 of growth. The ova and spermatoblasts represent the two forms 

 of germinal cells which have become free, and which, after a mutual 

 interaction in the process of fertilization, develop into a new 

 organism. Nevertheless under certain conditions the egg can, like 

 the simple germ cell, undergo spontaneous development ; numerous 

 instances of this mode of development, which is known as partheno- 

 genesis, are found in Insects. The necessity of fertilization therefore 



\ 

 Hr 



FIG. 99. Viviparous form of Aphis platanoides. Oc, ocelli ; Hr, honey tubes. 



no longer enters into our conception of the egg-cell, and no absolute 

 physiological test is left to enable us to distinguish it from the germ- 

 cell. It is usual to regard the place of origin in the sexual organ 

 and in the female body as a feature distinguishing the ovum from a 

 germ cell, but even with this morphological test we do not in each 

 individual case arrive at the desired result (Bees, Bark-lice, Psyckidce). 

 We have already given prominence to the fact that ovaries and 

 testes, in the simplest cases, consist of nothing more than groups 

 of cells of the epithelium of the body cavity or of the outer skin. 

 These, however, do not acquire the character of sexual organs until, 

 at a higher stage of differentiation, the contrast between the 'two 



