244 PHYSICAL BASIS OF HEEEDITY 



males are always hen-feathered. This means that the 

 feathers of the neck and back and the tail coverts of the 

 Sebright cock are nearly like those of the hen of this 

 breed, and not long and pointed as in the ordinary cock. 

 When Sebrights are crossed to game bantams (which 

 have ordinary males), the F^ males are hen-feathered. 

 "When these are inbred the two types reappear in the F^ 

 males. One, or probably two, Mendelian factor differ- 

 ences account for the results. 



It has been shown that when the testes are removed 

 from the Sebright male, he then develops at the next moult 

 (or at once if some feathers are plucked out) the long 

 and highly colored feathers of the ordinary male (Fig. 

 107, h). It is probable, therefore, that the testes of the 

 Sebright produce an internal secretion that inhibits in the 

 male the full development of certain feathers. This makes 

 him like the hen, and in this connection it is interesting to 

 note that when the ovary of a hen of an ordinary breed is 

 removed she also develops the full plumage of the cock, as 

 Groodale has clearly demonstrated. Whether the testes of 

 a male are of the sort to develop this inhibiting substance, 

 depends on the presence in the cells of the testes of certain 

 genetic factors. These factors are present, presumably, 

 in all the cells of the body, but if they are, their activity 

 is ineffective in the absence of secretions produced by the 

 testes, as is shown by the castrated Sebright becoming 

 cock-feathered. Whether this substance belongs in the 

 heterogeneous group of substances called hormones — 

 defined by the kind of action they produce rather than by 

 any chemical peculiarity — or to the groups. of enzymes 

 that have a more or less specific action, cannot be stated. 



The foregoing discussion touches upon the question as 

 to whether there is any evidence that the genes themselves 

 are to be regarded as enzymes.* In almost all of the 



* Inadequate as is our knowledge of the physico-chemical processes that 

 go on in development, it is enough to indicate that many processes are 

 at work. 



