74 EDWARD DRANE CRABB. 



IV. SELF-FERTILIZATION AND PARTHENOGENESIS. 



A number of investigators have isolated pond snails more or 

 less carefully, and since they secured offspring they concluded 

 that their snails reproduced parthenogenetically or by self-fertili- 

 zation. Some investigators have tried to discriminate between 

 self-copulation and internal fertilization, the latter being insemina- 

 tion that occurs in the hermaphrodite apparatus or uterus without 

 self-copulation. Kunkel ('08) thinks that both internal self- 

 fertilization and self -copulation occurs in L. stagnalis. Braun 

 ('88) reared L. aurlcularia in isolation, and these produced off- 

 spring. Although he is not certain that self-copulation occurred 

 in every case, he believes that self-fertilization took place. Colton 

 ('22) self-fertilized L. coluuidla for forty-seven generations. 

 A. Lang ('oo) thinks that self-fertilization without self-copula- 

 tion occurs in Lynuura; while Semper (Braun, '88) thinks that 

 in all cases of reproduction in isolation self-copulation takes place. 



The only investigators claiming parthenogenesis for fresh- 

 water snails appear to be Robson ('23, '26) for Paludestrlna 

 jcnkinsi and Pelseneer ('19) for L\nincca. The latter states that 

 in isolation cultures the eggs of L. auricularia, L. glntinosus and 

 L. pahtstris give off only one polar body. 



In my L. palustris isolated individuals lay eggs which extrude 

 two polar bodies regularly. Colton ('18) states that in the eggs 

 of isolated L. coluinclla, two polar bodies are normally given off. 

 and Kunkel (Diver, '25, p. 125) shows that two polar bodies are 

 extruded in the eggs of isolated Arion and Lima.v. Thus Colton 

 believes that isolated pond snails self-fertilize, while Pelseneer 

 thinks that they reproduce parthenogenetically in isolation and also 

 when mated with another species. In the latter case Pelseneer 

 believes that the sperms from an individual of another species 

 merely stimulate the egg to develop parthenogenetically, as Belaf 

 ('24) has described in two forms of the nematode Rhabditcs. 

 Els where (Crabb, '27) the writer has pointed out that breeding 

 experiments indicate that self-fertilization is the normal method 

 of reproduction in pond snails. 



V. PROTANDRY. 



The question whether L. s. apprcssa is protandrous or proto- 

 gynous has been raised. I have sectioned several young snails 



