Forms Having an Extra Chromosome. 27 



have 15 chromosomes. De Vries showed (1909, Vol. I, p. 244) 

 that this form also behaves in a peculiar way in inheritance, usually 

 giving rise to an average of about 57% Lamarckiana, 32% scintillans 

 9% oblonga and 1% lata. In one family as much as 69% scintillans 

 was obtained. It is significant that several of the original scintillans 

 mutants were from lata pollinated by Lamarckiana. From these 

 results it would appear that scintillans originates from the union of 

 two germ cells, one carrying 8 chromosomes and the other (or 

 perhaps the same one) carrying the oblonga qualities. On the other 

 hand, it is not impossible, though very unlikely, that the oblonga type 

 to which it gives rise, has 16 chromosomes, as Miss Lutz (1917) 

 suggests. Also, while lata had a mutation frequency in de Vries' 

 cultures of 0-413%, the frequency of scintillans was only 0-038%. 



As predicted in 1915, 1 the cytological study of scintillans, 

 resulting in the determination of an extra chromosome in its nuclei, 

 throws much light on its origin and behaviour. It seems reasonable 

 to interpret scintillans as arising from a germ cell with an extra 

 chromosome + one bearing the oblonga characters, just as we picture 

 lata arising from a germ cell with an extra chromosome + one 

 having the typical Lamarckiana characters. This will depend, 

 however, on the chromosome counts in the various oblonga types, 

 which are at present uncertain. That the offspring of scintillans 

 were always found to include 1% or more lata, shows a close 

 relationship between these two types, and indicates, though it does 

 not prove, that the extra chromosome may be the same one in both 

 cases. Another alternative is that any chromosome, if present in 

 duplicate in a germ cell owing to non-disjunction in the reduction 

 division, will give the lata features if combined with a Lamarckiana 

 germ cell or scintillans if combined with an oblonga germ cell. 

 This is improbable for many reasons. 



In regard to the relation between these forms, lata, semilata, 

 and scintillans, we thus appeared at one time to be left with only 

 two alternatives, either that the extra chromosome is the same in 

 every case, as the breeding data suggest, 2 or that any of the 7 

 haploid chromosomes when duplicated will give the same 

 morphological result. The latter alternative is highly improbable 

 for general cytological reasons. However, the frequency of lata is 

 apparently greater in the offspring of scintillans than of Lamarc- 

 kiana, and it is conceivable that the unbalanced condition of 

 scintillans may lead to more frequent chromosome irregularities. 



1 The Mutation Factor, p. 142. 



1 It seems highly probable that in lata and semilata at least, the extra 

 chromosome is the same. 



