32 On the Effects of Castration and Ovariotomy upon Sheep. 



It is just possible, therefore, that the development of scurs in the first ewe 

 and their doubtful development in the two ewes that are still alive, may 

 have been due to the removal of the ovaries, which on this view may be 

 regarded as organs exercising an inhibitory influence on horn growth. In 

 other breeds {e.g. Suffolks), where scurs frequently grow in the ram, these do 

 not generally (if ever) make their appearance until the second year, and it 

 is conceivable that in the two spayed ewes which are still being kept, definite 

 scurs may manifest themselves at a later age, though this seems unlikely, 

 ■since the animals are now 19 months old. (Compare, however, Goodale's 

 duck, referred to above.) It has already been mentioued that, according to 

 information supplied me, scurs occasionally grow in Herdwick ewes. In 

 view of this fact it would appear that the only conclusive way of determining 

 ■definitely whether scurs can be induced to grow in Herdwick ewes as a result 

 of ovariotomy would be to spay a considerable number of individuals and 

 ■compare these with an equal number of unspayed sheep. Lastly, it is always 

 possible that the presence of the ovaries in early life, or even before birth, 

 may exercise a permanently inhibitory influence which can last after the 

 operation of removal. 



Conclusions. 



(1) The development of horns in the males of a breed of sheep in which 

 well marked secondary sexual differentiation occurs (as manifested especially 

 by presence or absence of horns) depends upon a stimulus arising in the 

 testes, and this stimulus is essential not merely for the initiation of the horn 

 growth but for its continuance, the horns ceasing to grow whenever the 

 testes are removed. 



(2) The removal of the ovaries from young ewes belonging to such a breed 

 does not lead to the development of definitely male characters, except 

 possibly in an extremely minor degree. 



The work was carried out at the Field Laboratories, Milton Eoad, 

 Cambridge. The expenses were partly defrayed by a grant from the 

 Government Grant Committee of the Eoyal Society, to whom I am under 

 obligations. I wish also to express my indebtedness to Mr. G. A. Banham 

 and Mr. J. G. Eunciman for their kind assistance in carrying out the 

 experiments. 



