t68 SEX-DETERMINATION [CH. 



divide equationally. In the second spermatocyte division 

 the autosomes divide equationally, but the two ^-chromo- 

 somes pair and separate towards opposite poles. They lag 

 behind the autosomes, and according to SCHLEIP one is 

 included with the daughter-nucleus at one pole, while the 

 other lags so much that it remains outside the nucleus in 

 the telophase and is ultimately extruded from the spermato- 

 zoon with the " Spindelrestkorper." As a consequence, half 

 the spermatozoa have an .AT-chromosome and half are with- 

 out it. BOVERI describes the second spermatocyte mitosis 

 rather differently; he believes that the two J^-chromosomes 

 may either go both to one pole, giving spermatids with 

 seven and five chromosomes (two X and no ^Y), or may 

 separate from one another and go one into each spermatid, 

 giving six in each. He supposes that the spermatids with 

 seven are functionless, that those with six cause the fer- 

 tilised egg to become a female, and those with five, a male, 

 of the free-living generation. No explanation has been 

 offered of the fact that fertilised eggs with twelve chromo- 

 somes become females in one generation and hermaphrodites 

 in the next, nor of what causes the different behaviour of 

 the Jf-chromosome in the oogenesis and spermatogenesis of 

 the hermaphrodite. 



Another hermaphrodite species in which sex-chromo- 

 somes have been described is the Pteropod Creseis investi- 

 gated by ZARNIK. He found a difference between the 

 oogenesis and spermatogenesis to some extent comparable 

 with that observed in Rhabdonema, but his results are in 

 some respects obscure, and until the case has been examined 

 again it is hardly possible to accept them as final, and in 

 any case the cytological basis for the production of male and 

 female germ-cells by one individual remains unexplained. 

 Further investigation of the whole subject is greatly needed. 



