410 Proceedings of the Royal Physical Society. 



immediate ancestors." He makes no remarks, however, upon 

 the evolution of the follicle. 



Poulton is led by his results to the belief that " there are 

 important distinctions between the Graafian follicle of a 

 marsupial and those of the higher mammalia, and yet these 

 distinctions are by no means in the direction of greater 

 simplicity in the former, but rather the reverse." And 

 again he expresses the opinion that the structure of the 

 marsupial follicle shows no indications of any transition 

 between the Monotremes and Placentalia. 



The function of the follicular epithelium is universally 

 admitted to be a nutritive one. Limiting ourselves to the 

 mammalia, Heape and Caldwell have demonstrated the 

 continuity of the follicular cells with the ovum by means 

 of fine processes which perforate the Qgg membrane. And 

 this has been shown to be the case by Balfour and others 

 in other groups of the animal kingdom ; there is, however, 

 some difference of opinion as to condition in which the 

 nutritive matter is passed from the cells of the follicle to the 

 ovum. Caldwell agrees with others in the belief that (in 

 Monotremata and Marsupials) yolk particles are formed first 

 of all in the cells of the follicle and then pass into the 

 ovum, where yolk is also independently formed. Balfour 

 appears to have held the opinion that the nutritive matter 

 is not yolk but protoplasm in an intermediate condition — a 

 stage nearer to yolk. It is possible that both processes occur. 



The marsupial and placentalian follicles are of about the 

 same size as the monotreme follicle ; ^ Caldwell mentions 

 3 mm. as the diameter of a fully developed ovarian Qg^\ 

 and a minute fraction must be added to this for the thick- 

 ness of the follicular epithelium; in Qiiain's "Anatomy," 

 it is stated that the Graafian follicle of the human female 

 reaches a size of \ inch = 4 mm. in diameter. 



As, however, the ovum is so much smaller in the higher 

 mammalia, it follows that it does not require so much 

 nutritive matter ; hence the activity of the follicular cells, 

 inherited from the Monotreme ancestor, is spent chiefly in 

 rapid proliferation; at first, when the ovum had shrunk 



1 In Phascolarctos, however, the follicle is much larger (Caldwell). 



