246 Hybrids of Bistonine Moths 



sterility. It is true that Federley's Pygaera hybrids were not sterile, 

 but that may be because little or no pairing took place, so that the 

 resulting gametes would be less abnormal than in a case where some 

 chromosomes pair and others do not. 



The second point mentioned at the beginning of this discussion was 

 the one which led in the first instance to my undertaking the work — 

 the possible cause of the fact that only males are produced from the 

 cross zonaria $ x hirtaria ^. Similar results have been obtained 

 with other species by various observers, notably the cross Tephrosia 

 histortata % x T. crepuscularia fj described by Tutt^ Brake and 

 Goldschmidt's observations on Lymantria dispar and L. japonica may 

 also be compared, in which they find that dispar $ x japonica <^ gives 

 males and gynandromorphs, although the converse cross gives normal 

 males and females I As Mr Harrison has shown, true hermaphrodites 

 occur in certain crosses with the genus Biston, but not with those used 

 in the present case. 



My observations on the chromosomes do not yet give any conclusive 

 results with regard to the cause of these sex-phenomena, but there are 

 certain indications which perhaps deserve mention. It is probable that 

 in Lepidoptera there are two similar " sex-chromosomes " in the male, 

 and that one of these differs from the other, and may be regarded as 

 lacking the male determiner, in the female. This has been suggested 

 on the ground of the facts of sex-limited inheritance {Ahraxas)\ the 

 suggestion is supported, though not proved, by the observations of 

 Seiler, of which only a preliminary account has been published^; and 

 I have obtained additional evidence for it from a study of the chromo- 

 somes in a strain of Ahraxas which in each generation produces families 

 consisting only of females. In a first account of this work* I have 

 shown that females of this strain have 55 instead of ^Q chromosomes, 

 and further work, an account of which I hope will be published shortly, 

 confirms this observation, and shows that all spermatozoa have 28 

 chromosomes, while eggs have either 28 or 27. 



In B. hirtaria, although there is no certain indication as to which 

 chromosome is the *'' sex-chromosome," I think it may be assumed with 

 some probability that it is one of the larger ones. One of the largest 

 is always coupled with a small one in the first spermatocytes and such 



1 J. W. Tutt, Trans. Entom. Sac. 1898, p. 17. 



2 R. Goldschmidt, Zeitschr. f. indukt. Abstamm. vii. 1912, p. 1. 



3 L. Seiler, Zool. Anz. xli. 1913, p. 246. 

 ** Journ. of Genetics, iii. 1913, p. 1. 



