RELATING TO SECONDARY SEXUAL CHARACTERS. 33 



mammal the yellow corpus luteum is a large, gland-like organ that 

 develops after the ovum is discharged ; in the bird there is also a yellow 

 spot on the ovary, due to the pigment in the collapsed follicle, but it is 

 smaller and much less conspicuous than in the mammal. The evidence 

 concerning luteal cells in the testes of the bird is conflicting. One of 

 the difficulties in the situation is the identification of the cells, which 

 are sometimes regarded merely as the general connective-tissue 

 stroma of the testis that is undoubtedly present ; at other times special 

 secretory cells are discerned embedded in the connective tissue, as 

 individual cells or in islands. Boring states (1912) that in newly 

 hatched chicks about half of the tissue of the testes is interstitial con- 

 nective tissue; the other half consists of tubes or cords whose principal 

 function is the development of the germ-cells. In the paper of 1912 

 Boring reached the conclusion that there are no "interstitial cells in 

 the testes of the domesticated chicken in the sense that this term has 

 been previously used," and states that no evidence has been found that 

 an internal secretion of any kind is formed by any cells of the interstitial 

 tissue. 



It is not necessary to discuss whether or not connective-tissue cells 

 are present in the testes of birds, for is it generally conceded that they 

 are found at least in certain stages, but it is important to look into the 

 question as to whether among these interstitial cells there are others 

 that have an endocrine function. Mazzetti gives pictures of such 

 gland-cells between the seminal tubules of the cock bird, but says that 

 they are rare, "even though this bird has very marked secondary 

 sexual characters " (Boring and Pearl). It maybe remarked parenthet- 

 ically that if they had been more abundant the bird might have had 

 no secondary sexual plumage since it will be pointed out below that such 

 glandular cells may have as their special function the suppression of 

 these characters. 



According to Des Cilleuls, interstitial cells are first found in males 

 about 30 days old and at this time the secondary sexual characters put 

 in their appearance. If, as will be shown in the sequel, he means by 

 interstitial cells the endocrine cells that suppress the development of 

 the male plumage in the female, the appearance of these cells at this 

 time would be significant; but if he implies that their occurrence in 

 the male incites the development of the secondary sexual characters, 

 his interpretation is open to serious doubt. Reeves found interstitial 

 cells in testes of cocks 3, 5|, 9, and 18 months more in the earlier 

 stages. 



In a later communication by Boring and Pearl the whole question 

 is taken up again with improved methods, etc. Previously 21 male 

 birds had been studied, just hatched to 12 months old. More sec- 

 tions of this same material were made which were stained according 



