D. W. Cutler 161 



From the cross P. reevesii $ with P.formosanus (/ a sterile male was pro- 

 duced, and a second one from the cross P. reevesii $ with P. versicolor </. 



On examination of the testes of these hybrids it was found that 

 the spermatogonia! cells were normal, but all subsequent stages were 

 abnormal, owing to the failure of the two Sets of chromosomes to pair 

 during synapsis. 



On another occasion from the same crosses three sterile females woi-e 

 obtained. The ovaries of two of these were investigated, but there was 

 no trace of sexual cells, the whole oi'gan being composed of interstitial 

 cells mixed with stroma and fibrous tissue. 



The sterility of the male hybrids is therefore due, as in other cases 

 enumerated above, to the inability of the chromosomes derived from the 

 two jjarents to co-operate during maturation. 



Although in the female hybrids degeneration of the ovaries occurred 

 before maturation, it is probable that here also sterility is assignable to 

 the .same cause as in the males : for as G. Smith points out, in his paper 

 on pigeon hybrids, there is evidence that a precocious temporary synapsis 

 takes place in the female germ cells. Probably it was at this period that 

 the chromosome.s failed to pair, thus leading to degeneration of the gonads. 



A paper by Wodsedalek dealing with the sterility of the mule has 

 appeared recently. He finds that the maturation divisions are further 

 suppressed than in the above cases, for there is no trace of normal 

 synapsis, and the spireme condition which usually follows it is entirely 

 lacking. The conflict between the paternal and maternai chromosomes 

 begins in the primary spermatocytes leading tothe production of abnormal 

 cells. The spermatogonial chromosomes are 51 in number, but in the 

 spermatocytes the number varies from 34 — 49. Thus it apjDears that 

 there is some attempt on the part of a few chi-omosomes to pair. Wodse- 

 dalek found, however, that the most pronounced signs of decay were to 

 be seen in the cells where pairing had been greatest. 



Finally there is the case of sterile moths, h^'brids of the genus Bis- 

 toninae, which has been investigated by Harrison and Doncaster. The 

 parents of the hybrids were Biston hirtaria and Nyssia zonaria. Recip- 

 rocal crosses were made but the resulting offspring were all sterile. An 

 interesting change in the sex ratio was produced according as to the 

 manner in which the cross was made; this will be dealt with later in the 

 paper. In both spermatocytes and oocytes the first stages in synapsis are 

 normal, but there is no production of a thick coiled spireme. Complete 

 pairing of the chromo.somes does not occur and only a few are able to find 

 mates. It was estimated that in the cross zonaria female with hirtaria 



