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 odgonia and sperma- 
togonia of the hermaphroditic parasites possess 
twelve chromosomes (Fig. 73, 4). 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 (F) 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- 
