INTERNAL SECRETION OF TESTICLE 195 



also by Guyer (1912) in hybrids between the guinea and the 

 common fowl, and by Poll (1920 a, 1920 b) in hybrids between 

 pea and guinea fowl. It would be difficult to say whether 

 the hybrids showed a "normal" plumage or a "normal" 

 sexual behaviour, both having been different from those of any 

 other race ; only a detailed analysis and experimental investi- 

 gation could give evidence on this point. But it seems that 

 in the cases of both Guyer and Poll the sexual behaviour of 

 the hybrids was different from that of the ordinary "castrate," 

 and that there was consequently an internal secretion of the 

 testicle. 



We have no detailed knowledge of the changes the testicle 

 of the cock undergoes after transplantation. According to 

 Foges and Pezard the seminiferous tubules in the engrafted 

 testicle of the cock do not degenerate. Also Massaglia found 

 in one case numerous spermatozoa in the seminiferous tubules 

 in small fragments transplanted on to the peritoneum; the 

 interstitial cells were normal. The bird showed diminished 

 sexual instincts; Massaglia is disposed to explain this by the 

 small number of interstitial cells which were present in the 

 body. It may be that the quantity was so small that the effect 

 produced was beneath the threshold, and that the bird was 

 on the way to become a capon. 



As to the function of the interstitial cells in birds the ex- 

 periments of Cent (1909) on drakes should also be recorded here. 

 Cent (quoted from Boring and Pearl, 19 17) removed the cere- 

 bral hemispheres from drakes ; the interstitial tissue increased, 

 whereas the seminal tubules underwent a decrease. The sex 

 characters remained undeveloped. He does not state whether 

 this hypertrophied interstitial tissue consisted of connective 

 tissue or of epithelioid interstitial cells. The "interstitial 

 tissue " is a very complex formation, in which cells of Leydig are 

 intermingled with simple connective tissue cells. More 

 "interstitial tissue" does not imply more active epithelioid 

 glandular cells of the type of Leydig, and by ignoring this 

 fundamental consideration the discussion of the problem be- 

 comes impossible. 



We have some further knowledge of the interstitial cells 

 derived from the study of other birds such as the sparrow, 

 canary, pheasant and jackdaw. The data accumulated concern 

 the cyclic changes in the testicle occurring during heat, when 



