On the Structure of the Graafian Follicle in Didelpbys. 409 



If I understand Mr Poulton rightly, he wishes to compare 

 the central substance with the liquor foUiculi of the ordinary 

 mammalian follicle ; but to insist upon its difference in 

 structure, and upon the fact that the ovum is imbedded in it 

 directly through the gradual metamorphosis and disappear- 

 ance of the surrounding cells which form the liquor. 



More recently ^ Mr Caldwell has dealt with the same sub- 

 ject ; his observations refer to the ovary of Phascolarctos 

 cinereus ; the originally single-layered follicle becomes 

 several rows deep, and a cavity appears between the cells ; 

 this ultimately spreads all round the ovum, which comes to 

 lie isolated in the centre of the follicle, but surrounded by an 

 epithelium several layers deep ; the liquor foUiculi contains 

 numerous branched cells, which connect the epithelial lining 

 of the follicle with that surrounding the ovum. 



Finally, I have to record that in the opossum {Didelphys) 

 the structure of the follicle is slightly different from that of 

 Phascolarctos; it shows, in fact, no differences that I could 

 detect from the ordinary mammalian Graafian follicle ; the 

 ovum lies eccentrically, and is surrounded by a single layer 

 of follicular cells ; this is connected by a bridge of cells with 

 the layers which line the follicle ; the interspace is occupied 

 by the liquor foUiculi, which appears to resemble the liquor 

 foUiculi of placental mammals generally. 



Didelphys therefore agrees more closely with Phascolarctos 

 than with Phalangista. 



Now it is probable, on a priori grounds, that the follicle 

 of the Marsupialia would indicate a stage in the evolution of 

 the typical mammalian Graafian follicle from the primitive 

 Graafian foUicle of the Monotremata. Caldwell has pointed 

 out in the memoir referred to that the ovum in Phascolarctos 

 is to a certain extent intermediate between the large yolked 

 telolecithal monotreme ovum and the alecithal placental 

 ovum ; this is seen in the phenomena of segmentation ; the 

 first two segmentation furrows fail to divide the ovum ; and 

 this " shows that although the ovum has nearly regained its 

 original alecithal condition, it still retains the secondary 

 arrangement of protoplasm induced by the yolk of its more 



1 Phil. Trans., 1887. 



