SYNGAMY AND SEX IN THE PROTOZOA 



137 



syngamy occurs once for each complete cycle of development 

 with alternation* of hosts, though it must not be forgotten 

 that the development in the vertebrate host comprises a vast 

 and quite indefinite number of generations of the parasite. On 

 the other hand, in such forms as trypanosomes, a complete life- 

 cycle, from one act of syngamy to the next, may comprise, ap- 

 parently, a great number of developmental cycles and alternations 

 of hosts. 



From the foregoing considerations it is evident that syngamy, 

 though usually a necessity for the continued existence of uni- 

 cellular no less than of 

 multicellular organisms, 

 can be dispensed with for 

 a very large number of 

 generations, perhaps even 

 indefinitely, in some in- 

 stances or under special 

 circumstances. Two other 

 phenomena of apparently 

 widespread occurrence 

 point to the same con- 

 clusion namely, the phe- 

 nomena of parthenogenesis 

 and autogamy. Partheno- 

 genesis is a mode of re- 

 production so common in 

 Metazoa of various classes FIG. 72. Parthenogenesis of Plasmodium vivax. 



A, A female gametocyte, of which the nucleus 

 is dividing into a darker portion (n 1 ) and a 

 lighter portio"n (n 2 ); B, the separation of the 

 two parts is complete ; 0, the darker nucleus 

 has divided into a number of portions ; D, a 

 number of merozoites are formed from the 

 darker nuclei ; the lighter nucleus is abandoned 

 in the residual protoplasm (r.p.) containing 

 the melanin-pigment. After Schaud inn ( 1 30 ). 



that it is unnecessary to 

 cite instances of it here ; 

 it may be defined briefly 

 as the power to develop 

 without syngamy possessed 

 by a sexually-differentiated 

 gamete, which under nor- 

 mal circumstances could do so only after syngamy with a 

 gamete of the opposite sex. To this it must be added that 

 the gamete which has this power is always the female ; but this 

 limitation receives an explanation from the extreme reduction 

 of the body of the, male gamete and its feeble trophic powers, 

 rendering it quite unfitted for independent reproduction, rather 

 than from any inherent difference between the two sexes in 

 relation to reproductive activity. Parthenogenesis has been de- 

 scribed by Schaudinn for the human malarial parasite (Fig. 72) 

 and in Trypanosoma noctuce, and by Prowazek for Herpetomonas 

 rmiscce-domesticce ; none of these cases, however, are entirely free 



