90 PETER OKKELBERG 



the development of the second kind of germ cells may be the 

 result of a changed metabolism. 



In some true hermaphrodites sex conditions seem to be dis- 

 turbed at times so that true males and females appear. Accord- 

 ing to Maupas ('00), the number of males per thousand females 

 in various nematode worms may vary from 0.13 to 45. 



b. Alternation of the hermaphroditic and the dioecious con- 

 dition. A more complex case of hermaphroditism than those 

 mentioned above is that found in the nematode worm Rhabdites 

 (Rhabdonema) nigrovenosum, which is parasitic in the lungs of 

 frogs. While in the lungs, the worms are hermaphroditic, but 

 in the free-living state, which alternates with the parasitic, two 

 sexes occur. The free-living worms again give rise to herma- 

 phroditic parasitic offspring. This has been explained by Boveri 

 ('11) and Schleip ('11) as being due to the disappearance of one 

 kind of spermatozoa in the free-living males, so that upon fer- 

 tilization only one kind of sexual individual is produced, namely, 

 a female which again becomes parasitic. This female is capable 

 of giving rise to both spermatozoa and eggs, both of which should 

 have the same chromosomal make-up. During maturation three 

 kinds of spermatids are produced, with five, six, and seven 

 chromosomes, respectively. It is supposed that the last kind 

 degenerates. 



Some evidence has been advanced for a chromosomal explana- 

 tion of true hermaphroditism. Zarnik ('11) thinks that in cer- 

 tain hermaphroditic Pteropods, the female cells are of one kind 

 only (homogametic) ; while the male cells are of two kinds (hetero- 

 gametic), but that only one kind of male cells is functional, 

 namely, the one corresponding in chromosomal make-up to that 

 of the female cells. The offspring from such union should result 

 in a female, but instead it develops into a hermaphrodite in 

 which again half of the male germ cells degenerate. 



Kriiger ('12) found what she thinks is an accessory chromosome 

 in the hermaphrodite Rhabdites aberrans. During spermato- 

 genesis it becomes distributed equally among the spermatozoa, 

 with the exception of a very few cases when it lags behind and is 

 retained in one cell. Apparently in this species the spermatozoon 



