C. J. Bond 215 



Suggested explanatory hypothesis. 



The question now arises whether any explanation can be suggested 

 which will fit the facts in this and similar abnormal cases. Assuming 

 that the fertilized egg fi'om which this zygote developed was a female 

 egg, i.e. heterozygous in respect of sex, then we must suppose that at 

 the stage of blastomeric segmentation at which the division of the body 

 into two halves was laid down, the se.x factor (instead of passing in 

 equal portions to each half of the germ) divided unevenly, the factor 

 for raaleness passing into the left half and the factor for femaleness 

 into the right half of the body. This lateral segregation was not 

 however clear cut in those rudiments which represent the median 

 fusion body areas, while in the tail region this qualitative segmentation 

 was delayed till those cell divisions occurred which control the develop- 

 ment of the separate halves of each tail feather. 



The essential feature in this explanation is that it takes account 

 of genetic as well as physiological factors. It ascribes an abnormal 

 physiological phenomenon like the asymmetrical assumption of secondary 

 sex characters to the interaction of two factors, a peripheral or somatic 

 factor, and a central or sex gland gametic factor, though of course both 

 these owe their abnormal limitations to a common abnormality of genetic 

 origin, possibly arising at the time of the segregation between somato- 

 plasm and reserve germplasm in the zygote. 



In thus falling back on genetic factors one is forcibly reminded of 

 certain features in modern views of heredity. There also the associa- 

 tion between the gametic factor in the fertilized ovum and the unit 

 character in the zygote is pictured in just the same way. The 

 gametic factor stands to the unit chai-acter in the same relation that 

 the sex gland and its Hormone stand to the corresponding secondary 

 sex character. Just as we are still unable to decide whether the 

 appearance of any given secondary sex character is due to the presence 

 of a corresponding sex gland Hormone, or to the absence of an opposite 

 sex gland Hormone which, if present, might inhibit the influence of the 

 former, so students of heredity are still unable to decide whether any 

 dominant unit character, such as tallness in the pea, or an abnormality 

 like Brachydactyly, depends on the presence of a factor for tallness or 

 short fingeredness, or on the absence of another factor which, if present, 

 might inhibit the action of this abnormal factor, and so make for 

 normality. 



