D. W. Cutler 163 



of the chromosomes. Thus it seems that although the failure of the 

 chromosomes to pair and separate is not the direct cause of sterility, yet 

 it is often the manifestation of the antagonistic nature of the constituents 

 of the germ plasm. 



Up to this I have dealt entirely with sterility of hybrids: there is, how- 

 ever, a second type where sterility is found in a pure species of animal. 

 Two such examples were investigated by Doncaster and myself The first 

 was that of a sterile tortoiseshell tom cat. These animals are exceedingly 

 rare and, when produced, are usually sterile, or fertile to only a slight 

 degree. The colour is normally found only in the female and can be pro- 

 duced by mating a yellow male with a black female. The male offspring 

 are then black. The yellovv colour is sex-limited in inheritance and goes 

 only with female producing gametes. On occasion, however, this yellow 

 factor goes with male producing gametes and tortoiseshell toms result. 

 Such a cat we had in our possession and though repeatedly mated no 

 offspring were developed. Investigation of the testes demonstrated that 

 the sterility was caused by the complete failure of the spermatogonia to 

 form. Sterility produced in this way was also found in a hen-feathered 

 cock, extracted from the cross Sebiight hen with Hamburgh cock. Hen- 

 feathering is undoubtedly an inherited character, so that these two cases 

 suggest that, when a factor normally confined to the female, is received by 

 the male animal, there is a tendency for sterility to result. Though there 

 appears at present to be no connection between these cases and those of 

 sterile hybrids it is possible that there is a common basis to both. That 

 many of the factors which behave in a Mendelian way are chemical ones 

 seems to beextremelj' probable. If then a factor common to the constitu- 

 tion of the female is on rare occasions transmitted to the male, the general 

 metabolism of the sex cells may be so upset that they are unable to 

 perform their usual functions. In the same way in hybrids the com- 

 position of the germ cells of the parents may be so divergent, that when 

 the two sets of chromosomes are brought together in the nuclei of the 

 gonads, they are unable to co-operate during the complicated phases of 

 maturation but are still quite capable of performing the activities 

 characteristic of the nuclei of the various somatic cells, and also of some 

 germ cells so long as they are not brought into intimate contact with one 

 another. 



It is by no means uncommon to find that the sex ratio of hybrids 

 is unequal, as was discovered by Giiyer and Geoffrey Smith in hybrid 

 pigeons, by Mrs Haig Thomas in pheasant hybrids, and by Mr Lewis 

 Jones in the hybrids which I have been able to investigate. 



