THE DIPLOID CHROMOSOME COMPLEXES OF THE PIG 179 



age 17 millimeters, with one extreme measuring 9 millimeters 

 and the other 25 millimeters. The majority approach very 

 near the average length. Recalling that the longest chromo- 

 somes of the series average 8.9 centimeters, it seems justifiable 

 to consider the chromosomes below chromosome 40' in the 

 length series to be very nearly of even size in the majority of 

 cases. 



Discussion of the results ohiained by measuring the chromosomes 



The conditions in Oenothera scintillans. In order that the 

 metrical studies of the chromosomes of the pig may be better 

 understood a synopsis of similar work on Oenothera scintillans 

 (Hance '18) is given below. As the chromosomes in this 

 Oenothera were much less numerous than in the pig, more clear 

 cut and definite results were obtained, and they aided greatly in 

 the interpretation of the observations on the latter. 



1. In material collected from three generations of Oenothera 

 scintillans the somatic chromosome number varies from 15 to 21. 

 This variation is not correlated with any particular tissue, but 

 exists throughout the plant. 



2. The type or fundamental number of chromosomes is 15. 



3. The 'extra' chromosomes divide regularly. 



4. Each pair of the type (15-chromosome) group differs from 

 the next pair in the series by 9 per cent, or a shorter pair is 91 

 per cent the length of the next longer pair. This is apparently 

 a constant relation. 



5. The 'extra' chromosomes have been shown to be fragments 

 of the type chromosomes, and the total length of the chromosomes 

 in the various classes of cells has been found to be practically 

 identical. The fragments are very nearly of even length. 



6. With the aid of the paired conditions of the chromosomes 

 and of the relation between the pairs, it has been found. possible 

 theoretically to reunite the fragments with the chromosomes 

 from which they have separated. 



7. Fragmentation in the cells studied, has been confined largely, 

 but not exclusively, to the longer chromosomes. 



8. Fragmentation does not occur in the germ line. 



