CHAPTER VII 



SEXUAL PHENOMENA IN THE PROTOZOA 



" Die Bedeutung des Konjugationsaktes ist ein Verjiingung der ihn begehenden Tiere. 

 Durch diese Verjiingung erscheinen uns die aus der Konjugation hervorgehenden Individuen 

 sehr geeignet, zu den Stammvatern einer Reihe durch Theilung sich fortpflanzender Gene- 

 rationen zu werden, im Laufe welcher allmahlich ein Sinken der Lebensenergiesicheinstellt. 

 Letzter Umstand findet seinen Ausdruck darin, das die Grosse der Individuen mehr und 

 mehr sinkt sodass schliesslich eine minimalgrdsse erreicht wird, worauf eine neue Konjuga- 

 tionsepoche eintritt." BL'TSCHLi. 1 



THE power of the animal or plant to reproduce its kind from a por- 

 tion of its own body is bound up, in the higher forms of life, with sexual 

 processes and, in its more familiar forms, accordingly, is characterized 

 as sexual reproduction. It involves the union of two cells having quite 

 different characteristics ; the spermatozoon or male cell, being minute 

 and active, the ovum or female cell, larger and quiescent. Reproduc- 

 tion of the individual without sexual processes is, however, possible, 

 even in the higher organism, as we daily witness in plant " cuttings," 

 and as is almost equally well known in the higher forms of inverte- 

 brates such as insects, where, without . fertilization, an ovum may 

 develop into an adult form. In the lower forms of Invertebrata, such 

 as the worms and the Ccelenterata, so-called " asexual reproduction " 

 by division or by budding is widespread, and in the Protozoa this 

 method of reproduction is the usual form. 



The phenomena of parthenogenesis, or development from the egg 

 without fertilization, and reproduction by simple division or by bud- 

 ding as seen in the Protozoa and the lower Metazoa, have recently 

 led to the pertinent query : With what right do we distinguish the phe- 

 nomena of reproduction as "sexual " and " asexual " ? (Hertwig, '99). 

 In two recent publications R. Hertwig ('98, '99) has discussed this 

 question in a very interesting and convincing manner, and he main- 

 tains that, in order to speak of " sexual " reproduction, it must be 

 shown that in the Protozoa, for example, fertilization has some direct 

 effect upon division or that a certain specific form of division results 

 from fertilization, neither of which, he says, is true in the majority of 

 cases. 



All observers are apparently agreed that asexual reproduction as 

 seen in the processes of binary fission and spore-formation is a 

 result of growth, usually expressed by the statement that increase 



1 ('76), P . 421. 



211 



