VARIATIONS IN SOMATIC CHROMOSOMES. 35 



exist in a series of seven pairs plus one unpaired chromosome. 

 Each pair differs from the pairs longer and the next shorter pair 

 by almost precisely the same amount nine per cent. 



3. Since such exact relations exist between the normal pairs 

 alteration in any of the chromosomes would destroy these con- 

 ditions. This was found to be true in cells possessing more than 

 fifteen chromosomes. Comparison with the standard lengths 

 and relationships determined for the chromosomes of the normal 

 complexes made it possible to reunite, with fair accuracy, the 

 fragments to the chromosomes from which they had broken. 

 This reunion has shown that it was chiefly the longer chromo- 

 somes that were concerned in fragmentation. 



4. In the germ cells, the telophase chromosomes of the first 

 maturation division are favorable for study. Two classes of 

 cells are produced by this division, one of which has seven and 

 the other eight chromosomes. This is due to the fact that 

 the unpaired, and smallest, chromosome passes to one pole un- 

 divided. When the two complexes are measured it is found that 

 total length of their chromosomes differ exactly by the length of 

 the eighth chromosome, or the unpaired element. The same 

 percentage of length difference exists between the individual 

 gametic chromosomes as between the somatic pairs showing the 

 two sets to be entirely comparable. 



THE CYTOLOGICAL CONDITIONS IN THE PIG, Stts scrofa. 



The work on the cytology of this mammal was made possible 

 through an improvement in the preservation of the chromosomes 

 which gave, I believe, the first true picture of the conditions in 

 the cells of this group. Exception possibly should be made to 

 the material used by Winiwarter in his studies, although it seems 

 that the clear complexes were not as numerous as the other kind, 

 whereas in the present specimens good mitotic figures were ex- 

 ceedingly abundant. 



In the pig, I have found essentially the same cytological phe- 

 nomena as in (Enothera scintillans. The spermatogonial chromo- 

 some number is forty and this appears to be constant. In the 

 somatic cells the number ranges from forty to fifty-eight. As in 

 the plant the extra number is caused by fragmentation and not 



