408 Proceedings of the Royal Physical Society. 



paper contains a description of the microscopical appear- 

 ances of the ovary of Phalangista vulpina, which is all 

 the more reliable for being the outcome of a study of fresh 

 material. 



The ova themselves do not seem to differ materially from 

 the ova of other mammalia, except of course from the re- 

 markable ova of the Monotremata figured and described in 

 the same paper ; ^ but the follicle shows certain peculiar 

 features of interest, which, however, as I shall show in the 

 present paper, are not found in all marsupials, but are want- 

 ing in the opossum. Mr Poulton very naturally surmised 

 that his description of the ovum and its follicle in Phalan- 

 gista would " prove to be characteristic of the order." 



The youngest ova of Phalangista, as of mammals in 

 general, are surrounded by a single layer of flattened epithelial 

 cells. These multiply and form a " granulosa " several cells 

 deep. The first appearance of the liquor folliculi is perfectly 

 normal; it appears as a coagulated (by alcohol) mass between 

 the cells of the follicle on one side of the ovum ; between 

 this fluid mass and the follicle cells next to the limiting 

 membrane of the follicle is a layer of altered follicular cells, 

 which have become swollen, are unstained, and have no dis- 

 tinct nucleus. In later stages the whole follicle increases 

 enormously in size ; the ovum is in the centre of the follicle, 

 surrounded by a few layers of unaltered cells, and again 

 surrounded by a fluid mass separating it from the unaltered 

 cells lining the follicle peripherally. In the full-sized follicle 

 the ovum is imbedded in the fluid substance directly with- 

 out any trace of cells round it ; the fluid substance passes 

 gradually into the unaltered cells of the wall of the follicle 

 through an intermediate layer of altered cells. The appear- 

 ances seen in the different stages, in fact, suggest a gradual 

 change in the follicular cells, both centripetal and centrifugal, 

 which eventually results in their conversion into a gelatinous 

 mass in which the ovum is still imbedded, only a peripheral 

 layer remaining unaltered. 



1 The ovarian ovum of Echidna has been more recently described by Dr 

 Guldberg {Sitz. Ber. Jen. Gesells. f. Med. v. Naturw., 1885), and by myself 

 {Proc. Boy. Phys. Soc, 1885) ; and by Caldwell {Phil. Trans., 1887). 



