DESCRIPTION OF PLATE V— continued. 



made out when better material is obtained. It is seen that these structures 

 do not project from the surface to the same extent as the large follicles. 



Fig. 9. — x 188. A small follicle from the ovary of Ornithorhynchus. At 

 this stage the follicle is not at all unlike those of the higher Mammalia, but in 

 one of this size the follicular epithelium (see Fig. 4, g.f.) would be flattened, 

 and the zona pellucida less marked, f. e. Follicular epithelium composed of 

 cubical cells, resting upon a basement membrane, z. p. Structureless mem- 

 brane probably the zona pellucida, generally strongly adherent to the follicular 

 epithelium, o. Ovum, with n. the nucleus, containing a probably altered 

 uucleolus. 



Fig. 10. — X 188. A rather larger follicle. The references are similar, 

 except thaty. t., the fibrous tunic, is added in this case. 



Fig. 11. — X 188. The ovum in this instance had shrunk, but retained its 

 shape notwithstanding. The follicle was about twice the diameter of the ovum 

 (and the latter when uushrunk must have filled it), aud the interval was filled 

 with the debris of epithelium and zona pellucida. Two of the fragments are 

 drawn. The difference in size between the cells of this figure and the preced- 

 ing may be due to different degrees of contraction. The reference letters are 

 the same as those of the preceding figures. The nucleus {n) is seen to possess 

 a distinct limiting membrane, within which it has shrunk. The external part 

 of the ovum (o) is finely granular, even under high powers ; within this is a 

 zone chiefly composed of large granular spheres, and again in the centre the 

 ovum becomes finely granular. The nucleus is situated in the intermediate 

 zone, and is probably on its way to the periphery. Although the ova of the 

 specimen here described were very much altered and sometimes completely 

 disintegrated, yet I found the appearances so often and so distinctly that I 

 think it very probable that they represent to a large extent the actual con- 

 dition of the ovum. 



Fig. 12. — x 405. The periphery of a large follicle (probably 1 mm. in 

 diameter, and more when uncontracted) highly magnified, /. i. Fibrous tunic, 

 from which the epithelium (/. e.) aud its basement membrane {b. m.) have 

 shrunk away. z. p. Zona pellucida, on the inner surface of which is a thin 

 layer {z'. p'.), which may be the indication of a laminated structure, or may 

 be possibly an optical effect or a result of change. In some cases I have been 

 able to detect two such lines, v. m. A fine layer within the zona pellucida. 

 e. I. External layer of the vitellus, which stains deeply and is finely granular. 

 This is very constant, and adheres closely to the zona pellucida, so that it is 

 common to find short arcs composed of these two layers with the follicular 

 epithelium also attached to the outside of the zona pellucida, while the rest 

 of the ovum has disintegrated and the periphery has thus been broken, 

 y. s. Large finely granular spherical or oval masses. Between these and the 

 deeply-staining external layer {e. I.), just described, there is a finely graunlar 

 layer which does not stain deeply. 



