362 Proceedings of the Boyal Physical Society. 



(2.) The ovum completely fills up the follicle. 



(3.) The mature ovum is of immense size compared with 

 that of other mammals. 



(4.) The ovum is invested externally by a single membrane, 

 which has a more or less laminated structure, and 

 corresponds to the vitelline membrane ; there is no 

 zona radiaia present at any period (?). The vitelline 

 membrane atrophies or becomes extremely thin 

 when the ovum has attained maturity. 



(5.) The contents of the ovum are differentiated into a 

 finely granular peripheral layer, and a central por- 

 tion consists of larger and smaller yolk spherules 

 enclosed in the meshes of a fine protoplasmic net- 

 work. 



(6.) The germinal vesicle, with its germinal spot, lies 

 eccentrically, within the peripheral layer. 



Explanation of Plates, 

 plate xvl 



Transverse section tli rough a portion of the ovary of Echidna, magnified 

 about 18 diameters to show the relative size of ova in their different stages of 

 development : a, b, c, ova in various stages of development ; d, ovarian 

 stroma ; e, blood-vessel. 



PLATE XVIL 



f= Follicular epithelium. 

 7)1 = Egg membrane. 

 g.v. = Germinal vesicle. 



Fig. 1. Corpses luteum. a, yellow cells ; h, coagulum. 



Figs. 2, 3, 6, 7. Ova in various stages of development. In Fig. 7 most of 

 the yolk spherules are represented as having been removed. 



Fig. 4. A portion of a large ovum measuring 2 mm. in diameter indicated 

 on Plate lb ; a, b, c, yolk spherules. 



Fig. 5. Portion of ovum (Fig. 7) more highly magnified, to show the 

 protoplasmic reticulum enclosing the yolk spherules. 



Fig. 8. Portion of the germinal epithelium, egg membrane, and yolk of 

 ovum (Fig. 7). 



