xi] HERMAPHRODITISM 167 



Our knowledge of the behaviour of the sex-chromosomes 

 in hermaphrodite species is still far from complete. The 

 best-known case is that of the Nematode Rhabdonema 

 (Rhabditis) nigrovenosum studied by BOVERI (1911) and by 

 SCHLEIP (1911). This species has two generations in the 

 complete life-cycle, one of which consists of hermaphrodites 

 living as parasites in the lungs of the Frog, while the other 

 is free-living and consists of males and females. The somatic 

 chromosome number of the hermaphrodites and free-living 

 females is twelve, and the males have eleven ; that is to say, 

 the hermaphrodites and females have ten autosomes and 

 two Jf-chromosomes, and the males only one ^-chromosome 

 (PL XX). In the spermatogenesis of the free-living males 

 two kinds of spermatids are produced, one of which has six 

 and the other five chromosomes. BOVERI supposes that the 

 spermatozoa having five, and thus lacking the X-chromo- 

 some, are functionless, so that all fertilised eggs contain 

 twelve, and develop into hermaphrodites. The evidence, 

 however, for the degeneration of the spermatozoa without 

 an Jf-chromosome is at present very slender. In the her- 

 maphrodite generation the gonad consists of two parts, a 

 larger ovarian portion and a smaller testicular portion 

 which produces spermatozoa. The primitive germ-cells, 

 whether oogonia or spermatogonia, have twelve chromo- 

 somes. In oogenesis all these chromosomes behave alike; 

 before the polar divisions they unite in pairs to form six 

 bivalents, and the two maturation mitoses leave six chromo- 

 somes in the egg. In the spermatogenesis of the hermaphro- 

 dites, however, the two ^-chromosomes behave differently 

 from the others. Instead of pairing to form a bivalent they 

 usually remain separate; only rarely do they pair. In the 

 first spermatocyte division the bivalent autosomes separate 

 into their single components, while the ^T-chromosomes 



