On the Strudiore of the Oraafian Fullide in Didelpliys. 411 



comparatively little, the follicular layers would still form a 

 continuous coating to the ovum, completely filling up the 

 space between the ovum and the walls of the follicle; at 

 first sight there appears to be no reason why the follicular 

 epithelium should not go on increasing, as the ovum 

 diminishes in size, and that the typical mammalian follicle 

 should not consist of a solid mass of cells. But as the folli- 

 cular cells increase in number, those most distant from the 

 periphery of the follicle, and therefore from the blood capil- 

 laries, will not obtain a sufficient food supply, and will there- 

 fore degenerate. This process appears to me to have just 

 set in in Fhalangista ; the follicle of this marsupial nearly 

 represents the hypothetical intermediate condition between the 

 Monotremata and the higher mammals. Poulton speaks of 

 the gelatiuous nature of the mass formed by the degenera- 

 tion of the follicular cells, which distinguishes it from the 

 liquor foUiculi of the higher mammals. This seems to me 

 to show that the gelatinous substance is nearly, but not 

 quite, inert matter ; it is in fact a stage in the conversion of 

 the follicular cells into liquor foUiculi ; the viscidity of the 

 protoplasm still remains. On this view, however, it would 

 be difficult to account — not for the growth of the ovum, 

 because it can absorb nutritious matter from the layer of 

 follicular cells round it — but for the persistence of those 

 very follicular cells ; the follicular cells acting as an inter- 

 mediary between the blood and the ovum must be them- 

 selves nourished by the blood. Mr Caldwell, however, has 

 removed this difficulty by the discovery, that in Phascolarctos 

 the follicular layers immediately surrounding the ovum are 

 connected by bridges of cells with the peripheral layers of the 

 follicle ; the same thing may possibly occur in Fhalangista. 

 A struggle for existence^among the cells of the follicle deter- 

 mines which cells are to survive for this purpose. In 

 Fhascolarctos, however, the follicular liquid exhibits the 

 characters which that fluid exhibits in the higher mammals ; 

 it is an inert nitrogenous fluid. It is often stated that the 

 liquor foUiculi is shed out by the follicular cells ; but it is 

 necessary to assume for the present argument that it is pro- 

 duced by the direct metamorphoses of the follicular cells, 



