PARTHENOGENESIS 161 



others for different kinds of trypanosomes and for tjypanoplasma, a 

 closely allied form; none of the descriptions, however, are sufficiently 

 convincing to establish a life cycle, while numerous contradictory 

 accounts indicate the need of further careful and unprejudiced 

 research. 



With the exception, therefore, of the case of Trypanosoma noctuce, 

 the flagellates present few well-defined instances of sex differentiation, 

 but other examples might be cited in which fertilization is accom- 

 plished by the union of anisogametes. In Mastigella vitrea, Gold- 

 schmidt ('07) has shown that a small non-motile gamete unites with a 

 larger flagellated gamete (Fig. 48, p. 1 19), a condition which reverses the 

 ordinary process, where the resting cell is usually larger and possesses 

 the attributes of an egg cell. Anisogamous conjugation occurs also 

 in Bodo caudatus, Bodo lacertae, and Monas dallingeri, and among 

 many of the phytoflagellates, where in Pandorina morum and Eudo- 

 rina elegans sex differentiation is well established, but in other forms, 

 as chlamydomonas, size differences are quite facultative. 



Among the rhizopods the formation of anisogametes appears to 

 be widespread, especially among the fresh-water types. Schaudinn 

 ('03) and Elpetiewsky ('08) showed that minute but anisogamous 

 gametes are formed in centropyxis and arcella, the gametes in all 

 cases having nuclei derived from the idiochromidia (Fig. 47, p. 119). 



Fertilization by exogamy appears to be, therefore, the most wide- 

 spread and the most complicated of all methods of fertilization among 

 the protozoa, while in the higher types the process is accompanied 

 by well-marked maturation phases, approaching in complexity very 

 close to the reducing divisions and polar body formation of the higher 

 animals and plants. 



D. PARTHENOGENESIS. 



The processes of autogamy, as outlined above (p. 139), seem to have 

 many points in common with parthenogenesis or development of egg- 

 cells without fertilization. While the end result is undoubtedly the 

 same in both, a difference is implied from the fact that differentiated egg 

 cells, which normally develop after fertilization by a spermatozoon, in 

 parthenogenesis develop without such union. Parthenogenetic eggs, 

 therefore, are, in a sense, abnormal and may be interpreted as present- 

 ing a phenomenon of cenogenesis whereby the egg returns to a primi- 

 tive condition. Boveri ('87) suggested and Brauer ('93) confirmed 

 the suggestion in connection with the parthenogenetic eggs of Artemia, 

 that parthenogenesis is a result of the fertilization of the egg nucleus 

 by a polar body (Wilson, The Cell, p. 281). Such fertilization, as in the 

 case of autogamy, is brought about by the union of sister nuclei. In 

 11 



