192 MANUAL OF ELEMENTARY ZOOLOGY 



proteus it is not at present known to take place. Partheno- 

 genesis has been said to take place in certain Protozoa, as, 

 for instance, in the malaria parasite, but it is very doubtful 

 whether this statement is correct. At the same time, if, as 

 may well be, the multiple fission of Amceha proteus re- 

 sembles germ formation in that there is left behind in the 

 residual protoplasm something that corresponds to the 

 body substance of Metazoa, then this process has the 

 essential features of parthenogenesis, for in it energids of 

 the nature of germs develop without conjugation. 



In the Metazoa, sexual reproduction by means of ova 



and spermatozoa always takes place. In some 

 Be production there is also asexual reproduction, as we have 

 Metazoa. seen in Chapter XL, but this is never the case 



in the higher kinds, such as the frog. Par- 

 thenogenesis, however, occurs in animals as highly 

 organized as insects. 



We have now seen, in a survey of the reproductive 



processes of a number of animals, that they 

 t* 1 * Problem present a series, in some members of which, as in 

 Reproduction. Amoeba and certain strains of Paramecium, sexual 



reproduction is not known to occur, while in 

 others it alternates more or less regularly with asexual 

 reproduction, and in yet others, as in the frog, it replaces 

 the latter entirely. These facts may be partly explained as 

 follows. In an animal of low organization there can easily 

 arise a bud, or other product of fission, which needs but 

 little remodelling to repeat the parent body, and thus 

 asexual reproduction is a less complicated process than 

 sexual, with its development of the body from a simple 

 oosperm. In more complex animals, however, simple 

 methods of asexual reproduction are not possible, and in 

 such beings one advantage of the process disappears. But 

 sexual reproduction has also the handicap of requiring the 

 union of two gametes. Thus we have still to explain its 

 taking place side by side with an easier alternative and 

 eventually replacing the latter altogether. In considering 

 this problem we must note (a) that sexual reproduction in- 

 volves conjugation, (b) that in order to bring this about it 

 must, as we have seen, effect a parting of the undifferenti- 

 ated^r germ-substance from differentiated substance if these 



