INFLUENCE OF SYNGAMY 131 



been elucidated. In some instances, however, unless it is assumed that 

 syngamy must of necessity take place from time to time, it appears that 

 reproduction by simple binary fission is continued indefinitely. Such 

 an organism divides into two daughter individuals, and when these have 

 become fully grown, division again takes place. A simple life-cycle of this 

 kind is characteristic of the amoebae, and it is only interrupted by the 

 amoebae becoming encysted under certain circumstances. 



Within these cysts, which are purely protective in function, the 

 amoebae may or may not continue to multiply by fission. When condi- 

 tions again become favourable, the cyst is ruptured and the amoebae 

 escape to continue their multiplicative existence. Similarly, many 

 trypanosomes can be handed on indefinitely from one animal to another 

 by simple inoculation of infected blood. There appears to be a continuous 

 process of reproduction by binary fission without the intervention of 

 either syngamy or encystment. Under natural conditions, however, 

 direct transference from vertebrate to vertebrate, except in the case of 

 Trypanosoma equiperdu?n, does not occur, the life-history being varied 

 by alternate multiplication in a vertebrate and an invertebrate. As far 

 as is known at present, multiplication in both hosts is by continuous 

 binary fission, though some authorities assume that a syngamic process 

 will be found to occur in the invertebrate. When such a change of hosts 

 is obligatory, the parasite is said to require an alternation of hosts for the 

 continuance of its life-cycle. In the case of certain blood-inhabiting 

 Sporozoa (malarial parasites) the alternation of hosts is characterized 

 by the occurrence of asexual multiplication in the vertebrate and syngamy 

 followed by the production of sporozoites in the invertebrate. 



Until recently it was considered that the periodic occurrence of 

 syngamy was essentia] for the continued existence of the race. This view 

 was the outcome of researches conducted on ciliates by Maupas and 

 Eichard Hertwig. Thus it was fdiown that Paramecium caudatum, after 

 a varying period of multiplication by fission, proceeded to conjugate. 

 Calkins (1904) found that, if conjugation was prevented, the ciliates, 

 though they continued to reproduce, gradually weakened and died. 

 Similar results had previously been obtained by Maupas (1888, 1889) 

 in the case of Stylonychia pustulata and other forms. It was believed 

 that these experiments proved that a race would invariably die out if 

 conjugation did not occur. Enriques (1903), working with Glaticoma 

 scintillans and G. pyriformis, and Woodruf? (1917) with Paramecium 

 aurelia, proved that this was not the case. The latter observer (1925), 

 having commenced with a single individual, has carried on the culture 

 by separating the daughter individuals produced at each division for a 

 period of fifteen years, during which over 10,000 divisions have taken 



