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 may be remarked parenthetically 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, 5½, 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 sections of this same material were made which were stained according to Mann’s and Mallory’s methods. In addition, a whole new series of preparations was made. A few interstitial cells, i. e., granule containing-cells were found in newly hatched chicks, but not in any of the 60 mature birds examined. LUTEAL-CELLS IN THE TESTES OF THE MALE SEBRIGHT.