204 PROBLEMS OF FERTILIZATION 



gametes. But, as is well known, this is by no means 

 always the case, for there are both hermaphrodite 

 animals and plants in which self-fertilization is difficult 

 or impossible. Two problems have usually been con- 

 sidered together in this connection, viz., the problem 

 of compatibiHty in fertilization and the problem of 

 viability and vigor of the offspring of such fertiliza- 

 tions. The latter problem, although related, will not 

 concern us here. 



The problem of self-fertilization has not been very 

 widely investigated in the case of hermaphrodite an- 

 imals. In rhabdocoel Turbellaria reproduction by self- 

 fertilization is common; it is also stated to occur 

 occasionally in certain trematodes and cestodes in spite 

 of an elaborate apparatus for cross-fertihzation. Ohgo- 

 chaetes and pulmonates appear to reproduce exclusively 

 by cross-fertilization; but Braun (1888) and Colton 

 (191 2) have shown that individuals of the pond snail 

 Limnaea reared in isolation from the egg may produce 

 fertile eggs. As parthenogenesis is unknown in mol- 

 lusks it is almost certain that these eggs were self- 

 fertihzed. A. H. Cook reports a similar case for Arion 

 (Cambridge Natural History). In the parasite cirripeds 

 (Rhizocephala) reproduction is invariably by self- 

 fertilization (J. W. Smith, 1906), and the same is true of 

 certain free-living nematodes (Maupas, 1900; Potts, 

 1910). In both of the latter groups special arrange- 

 ments exist for insuring self-fertihzation. Among the 

 ascidians Cynthia and Molgula appear to be self-fertile, 

 at least to a considerable extent (Morgan, 1904), but 

 Ciona in the same group is self-infertile, at least to a 

 considerable extent, which appears to vary somewhat 



