296 Shull. 



chromosomes. On the other hand the X- chromosomes may contain one 

 or more factors for femaleness and the Y- chromosomes one or more 

 factors for maleness, the latter being epistatic. All of the empirical 

 phenomena would be fully met by any of these hjT[)otheses. 



In the present paper I have used the formulae XXFF = 9 and 

 XXFf = d", not because I consider these formulae better adapted than 

 the other two sets of available formulae, to express the observed results 

 in Lychnis, but because these formulae have been used more frequently 

 «than either of the other two sets for cases in which the female is a sex- 

 homozygote. The preponderant use of the FF-Ff scheme is due, I think, 

 solely to cytological phenomena observed in certain classes of animals, 

 but wliich are absent in others and which have not yet been observed 

 iu plants. I have shown, in fact, (Shull 1911) that the origin and 

 genetic behavior of the hei-maphrodite mutants favor one of the other 

 formulations, because it seems more in harmony with other known cases 

 of mutation to assume that the hermaphrodites result from a retrogressive 

 variation in a dominant male determiner than from a progressive variation 

 in a cryptomeric female determiner. 



It can be readily shown that the discovery of sex-limited characters 

 does not help us to reach a decision as to whether the female is positive, 

 negative or neutral. Sturtevant (1912), writing of Bryonia and 

 Lychnis, remarks that "in the absence of cytological evidence or sex- 

 linkage phenomena a chromosome interpretation would perhaps be out 

 of place." The cytology of Lychnis dioica was worked out by Stras- 

 BÜRGER (1910), and a sex-limited character is now available, but there 

 is still no way in which we can decide with any security just what 

 the sex-determiners in Lychnis are. There are 24 chromosomes in the 

 somatic cells of both males and females, and one pair of these are larger 

 than the rest, but no consistent difference has been detected between 

 the members of this pair in either sex. The sex-limited character can 

 be related to the other available genetic formulae just as well as to tlie 

 set of formulae used in the present paper. Thus, if we assume that the 

 female is a negative homozygote, XXmm, and the male XXMm, the broad- 

 leaf factor, B, must be desciibed simply as allelomorpluc to M instead 

 of coupled with F. The formulae would then be as follows: — 



XBm.XBm homozygous broad-leafed female, 

 XBm.Xbm iieterozygous broad-leafed female, 

 XBm.XbM broad-leafed male, 

 Xbm .XbM narrow-leafed male. 



