492 



The Determination of Sex 



sexes an X chromosome is eliminated from the germ cells during 

 a resting stage. He studied Sciara ocellaris, but there is no 

 reason to suppose that the behavior of this species and of S. 

 coprophila is different. That some elimination must occur ap- 

 peared from the fact that the zygote contains nine chromosomes 

 whereas the gonads of the early larval stages have only eight. 

 When oogenesis and spermatogenesis occur, only six autosomes 

 and two X chromosomes are present. This species has no limited 



Fig. 142. Method of chromosome elimination in Sciara ocellans. The 

 chromosome to be eliminated comes into contact with the nuclear mem- 

 brane and migrates through it apparently autonomously into the cyto- 

 plasm. (Courtesy of Dr. R. O. Berry in the Proceedings of the National 

 Academy of Sciences.) 



chromosomes. Genetic evidence indicates that the missing chro- 

 mosome is one of the two X chromosomes of paternal origin. 

 Berry has reported that the chromosomes are not eliminated from 

 the germ cells during their cleavage stages, as is true of the 

 somatic cells, but that a chromosome is eliminated after the germ 

 cells have moved into their position in the gonads. During this 

 migration and for a considerable period thereafter, the germ cells 

 are in the resting stage; but the chromosomes appear as definite 

 but diffuse bodies or prochromosomes (Fig. 142). During this 

 resting stage, one chromosome moves towards the nuclear mem- 

 brane and then apparently passes through it into the cytoplasm. 

 This peculiar action occurs approximately simultaneously in all 

 the germ cells of a given individual and at the same stage of 

 development in all individuals. After it has passed into the 

 cytoplasm, this eliminated chromosome remains there for several 

 days and then degenerates. 



