2 RAYMOND PEARL AND ALICE M. BORING 



considered in Study XI of this series), with varying degrees of 

 maleness and femaleness indicate the presence of some sex regu- 

 lating substance in birds. Is this substance entirely different 

 from the corpus luteum probably connected with it in mammals, 

 or is there a corpus luteum or its homologue in birds? An in- 

 vestigation of this question has been undertaken in this study. 

 We consider that we have successfully demonstrated the pres- 

 ence of the corpus luteum in the domestic chicken. Further 

 discussion of the bearing of this fact on the whole question of 

 secondary sex characters will be deferred until a later paper of 

 this series, which will unfortunately probably be delayed for 

 some time, as one of the authors (R. P.) has been called upon 

 by the government to turn his attention to practical problems 

 during the war. 



A careful examination of the ovary of a bird which has been 

 actively laying shows three kinds of structures: the yolks of 

 various .sizes indicating different stages of development, the 

 discharged follicles in various stages of regression, and the 

 atretic follicles or degenerating eggs of different sizes. These 

 are all easy to identify when they are large enough to protrude 

 far from the surface of the ovary, that is, when they are larger 

 than 2 or 3 mm. in diameter. Under this size, it is impossible 

 to distinguish the discharged follicle from the atretic. Both 

 of them show a yellow or orange spot in the center. The ques- 

 tion naturally arises whether these yellow spots are homologous 

 in structure and origin with the mammalian corpus luteum. 

 They never develop into a large mass like the corpus luteum of 

 the mammal. They have the color of the spots on the cow 

 ovary which indicate remains of old corpora lutea. In order 

 to interpret these yellow spots, a study has been undertaken of 

 the progressive and regressive changes in the cell structure of 

 egg follicles in different conditions, undischarged, discharged and 

 atretic. 



The material used came chiefly from four birds, an actively 

 laying Bantam, a Barred Plymouth Rock in the same condition, 

 an old Compine past the laying condition, and a guinea-hen 

 with a large ovary containing several large yolks. Material 



