Chromosome Eliviination 491 



involves one of the two X chromosomes that have been derived 

 from the male parent, thus leaving in the germ line of all eggs 

 one X chromosome which came from the mother and one which 

 came from the father. One or more of the limited chromosomes 

 must also be eliminated from the germ line as otherwise their 

 number would increase with each generation. 



Further behavior of the cells in the germ line after the elimina- 

 tion of the X chromosome is also of interest. Oogenesis is similar 

 to that found in most organisms. The chromosomes pair nor- 

 mally at zygotene and show no peculiarities. There are three 

 pairs of autosomes and two paired X chromosomes, and one 

 member of each pair is subsequently found in the egg. In species 

 with two limited chromosomes, these also pair and separate 

 in the same manner as the autosomes. The regular behavior at 

 oogenesis and at spermatogenesis, however, is quite in contrast. 

 In the primary spermatocyte there is no zygotene pairing, and 

 later only one pole is present instead of the more usual two. 

 When the chromosomes pass to the pole, only the autosomes and 

 X chromosome of maternal origin pass to the single pole; but all 

 the limited chromosomes that may be present also pass to this 

 pole. The paternal X chromosome and the three autosomes 

 which had been contributed by the male parent move away from 

 the pole and become pinched off from the egg, and in that way 

 they are eliminated from further activity. In the division of 

 the secondary spermatocyte, the achromatic figure is bipolar, but 

 one pole develops before the other. All the chromosomes "split" 

 normally except one, which evidence indicates is the X chromo- 

 some. This X chromosome moves towards the first pole formed 

 before the daughter halves of the remaining chromosomes sepa- 

 rate; it therefore has been termed the "precocious" chromosome. 

 When the second pole forais, the daughter halves of all but the 

 precocious chromosome separate in the usual manner; and the 

 halves of the X chromosome separate from one another but 

 remain at the same pole. The group of chromosomes which lacks 

 the X chromosome then degenerates, and the other group with 

 the halves of the X chromosome remains functional. All the 

 sperms, therefore, are identical, and each contains only maternal 

 chromosomes. 



Berry has studied the elimination of an X chromosome during 

 the development of the germ cells and has found that in both 



