14 RAYMOND PEARL AND ALICE M. BORING 



degree of shrinkage shows whether the involution process had 

 recently begun or not. When these degenerating eggs are cut 

 open, the contents is found to be in a more or less fluid state. 

 When these atretic follicles have become reduced in size to 2 

 or 3 mm., it is no longer possible to distinguish them externally 

 from the discharged follicles; the same kind of a yellow pigment 

 appears in the center. 



Studied microscopically, the chief difference between atretic 

 and discharged follicles is that the former have a more distinct 

 cavity which becomes obliterated chiefly by migration of lutear 

 cells into it instead of by shrinkage of the walls. The granu- 

 losa is shed similarly. There must frequently be hemorrhage 

 as corpuscles are often found ia the cavity. The varying quan- 

 tity of yolk spheres is one indication of the degree of involu- 

 tion, also the number of lutear cells in the cavity. Figure 12 

 is an atretic follicle with considerable yolk still unabsorbed. 

 A few lutear cells have filled in to the cavity (fig. 13, 1). It 

 is particularly clear here that the cells inside of the inner mar- 

 ^n of the theca interna are the same in structure as those of 

 epithelial nature in the interna theca. This is just as Benthin 

 describes it for the atretic mammalian follicles. Figures 14 and 



15 show a later stage where the yolk is almost all absorbed and 

 the ca\dty is filled with lutear cells. 



Not until the cavity is filled with lutear cells does the yellow 

 pigment already described in discharged follicles, make its ap- 

 pearance. It forms in the lutear cells of atretic follicles in a 

 similar way to that in the discharged follicle. The cell bound- 

 aries are possibly not obliterated so completely, so that the 

 morphological resemblance to the cow corpus luteum remains is 

 even more striking than in the case of the discharged follicles. 

 Figure 16 is part of an atretic follicle where the cells are filled 

 with pigment. The amorphous character of this matedal 

 shows in figure 17 a part of figure 16 under higher magnification. 



It is of interest to notice that the lutear cells in the hen in 

 both discharged and atretic follicles originate entirely from the 

 theca interna. In mammals the origin of the lutear cells is a 

 mooted question. Some authors, as Niskoubina, hold that they 



