232 THE BIOLOGY OF BIRDS 



hen may be almost indistinguishable, being like an ancestral 

 form in which the sex-dimorphism was less marked. The 

 removal of the ovary from the hen allows the spurs and 

 plumage of a cock to grow. The removal of the testes from 

 the capon puts a stop to the development of the comb. 



O. Riddle has noted (1914) that female pigeons hatched 

 early in the season (when most males are produced) are more 

 masculine than their sisters hatched late in the season. Now 

 if extracts from the ovary of a pigeon be injected into those 

 masculine females, they come to behave like normal females. 

 Contrariwise, if testicular extract be injected into females 

 that are acting in a normal way, they come to behave as 

 males. This points to the conclusion that the sex-behaviour 

 of a bird is determined by internal secretions from the 

 gonads, and that these are carried by the blood to the central 

 nervous system. 



As to the precise source of the hormones that appear to 

 come from the reproductive organs, this is probably to be 

 found in " the interstitial cells " that are always present in 

 the ovary. The difficulty is in regard to the male bird, for 

 the results of good observers are very discrepant. Some 

 declare their presence in the testes, and others, as emphatic- 

 ally, their absence. Some discussion of the conflicting 

 results will be found in a paper by Alice M. Boring and 

 Raymond Pearl (191 7). These investigators found in the 

 testes of newly hatched chicks interstitial cells structurally 

 identical with those occurring in the ovary. But these 

 special cells are usually absent in males over six months 

 old which have reached full maturity, in respect of both 

 primary and secondary sex-characters. There is obviously 

 a difficulty here, and more facts are required. 



In the ovary of the hen there is a corpus luteum closely 

 resembling that in a mammal like a cow. It forms in the 

 place of discharged or degenerate follicles, and the cells 

 contained a yellow pigment and a fatty substance (AUce M. 

 Boring and Raymond Pearl, 191 8). It is beheved that 

 hormones pass into the blood from the corpus luteum and 

 are distributed through the body. 



