No. 535] NOTES AND LITERATURE 



427 



in their split condition tetrad groups, the eleventh being the un- 

 paired x element, as split a dyad. The x chromosome (dyad) 

 passes entire into one of the spermatocytes of the second order, 

 the end result being the formation of spermatids of two sorts, 

 those which contain six and those which contain five chromosomes. 



Now arises the first difficulty. If the male individual forms 

 two sorts of spermatozoa, why are not offspring of both sexes 

 produced by the free-living generation, instead of females alone? 

 Boveri finds no evidence of degeneration in the spermatozoa 

 containing only 5 chromosomes, and he finds that both sorts are 

 received by the female at copulation, but assumes that the 5- 

 chromosome sort is for some reason incapable of fertilizing the 

 eggs, because from these develop only females containing 12 

 chromosomes. He relies here upon an analogy with the case 

 of aphids and phylloxerans worked out by Morgan and von 

 Baehr. In those cases, namely, the spermatids with the smaller 

 number of chromosomes fail to develop. In the present case, 

 though developed, they fail, in Boveri 's opinion, to function in 

 the fertilization of the egg. 



Now comes the second difficulty. If the cells of the (hermaph- 

 roditic) female contain twelve chromosomes, how does she form 

 spermatozoa containing five chromosomes, which is less than the 

 half-numb,,-^ Boveri finds that it is by a peculiar method of 



equally at the maturation divisions. The egg accordingly always 

 contains 6 elements. But in spermatogenesis, in the same her- 

 maphrodite generation, there form 5 tetrads and in place of the 

 sixth a pair of separate dyads which are identified as x-elements. 

 These lag behind the tetrads in division, so that when the five 

 other elements have been distributed in cell-division these two 

 remain at the equator of the spindle. Boveri was unable to 



passes into half the spermatids, but the other remains on the 

 spindle and does not enter a spermatid. Hence only half the 

 Spermatids contain six elements, the others contain five. 



Why one process of reduction occurs in spermatogenesis and 



