CHROMOSOMES AND MITOCHONDRIA 269 



This nematode is a parasite in the lung of the frog 

 for part of its life cycle; during this period it re- 

 sembles the female, but is really hermaphroditic. 

 These hermaphrodites give rise to free-living indi- 

 viduals which are true males and females; the 

 eggs of the latter when fertilized develop into para- 

 sitic hermaphrodites. The oogonia and sperma- 

 togonia of the hermaphroditic parasites possess 

 twelve chromosomes (Fig. 73, A). The nucleus of 

 the mature egg is provided with six (B) . Two sorts 

 of spermatozoa are formed, one-half with six chromo- 

 somes, the other half with five ; the latter result from 

 the casting out of one chromosome (E) in a manner 

 similar to that described above in Phylloxera. The 

 eggs fertilized with the spermatozoa containing 

 six chromosomes (F) produce free-living, true fe- 

 males, whereas those fertilized by the spermatozoa 

 with five (G) develop into free-living, true males. 

 The hermaphroditic condition is regained as follows : 

 The free-living females give rise to eggs all with 

 six chromosomes ; the males, whose spermatogonia 

 contain eleven chromosomes, produce spermatozoa 

 with six or five chromosomes ; those with the latter 

 number, however, are not functional, hence all 

 fertilized eggs must be provided with twelve chromo- 

 somes and develop into the hermaphroditic parasites. 

 The chromosome cycle in pteropod mollusks as 

 worked out by Zarnik (1911) seems even more re- 

 markable than that described for nematodes. The 

 hermaphroditic species, Creseis acicula, possesses 

 twenty chromosomes, sixteen large ordinary chromo- 



