126 EDWARD B. POULTON. 



powers it is possible to make out a fine layer between the zona 

 and the third layer, and one (or sometimes two) fine lines in 

 the inner part of the zona, parallel with its surface. The 

 latter may be c.n indication of a laminated structure, or 

 it may be an optical effect, or caused by changes in the 

 tissues. The minute structure of these layers must be in- 

 vestigated with better material. The result of these facts is 

 that the ova of Monotremes practically fill their follicles, and 

 are of considerable size. In this specimen the ova had often 

 shrunk to half the diameter of the follicle, and the intervening 

 space was occupied by the loosely-packed debris of the epithe- 

 lium and zona. High powers showed that the follicular 

 epithelium rests on a thin but distinct raembrana propria, 

 which it often draws with it away from the follicular walls. In 

 young follicles the epithelium is thin, but it soon becomes cubi- 

 cal, and the zona appears, at first staining slightly, and of less 

 thickness than later ; always apparently homogeneous. Con- 

 cerning the ova, it is an obvious suggestion that in such large 

 structures there must be a well-marked distinction into food- 

 yolk and germinal-yolk. It is also probable that segmentation 

 is unequal, perhaps partial. The large ova were generally dis- 

 integrated, but from occasional specimens I found that the 

 nucleus (germinal vesicle) was eccentric, having been central in 

 small ova. The nucleus had a distinct limiting membrane, 

 within which it had shrunk (see fig. 11). There was a well- 

 marked nucleolus (germinal spot). Within the external, finely- 

 granular, deeply-staining layer of the ovum ; there was another 

 irregular layer of finely-granular material which did not stain 

 deeply, within which there were a number of various-sized 

 spheres and spheroids, some of them very large. These were 

 finely granular under high powers, and did not stain deeply. 

 Again, in the centre, there were traces of material similar to 

 that just external to the spheres (and which seemed to pass 

 between them). This may be the result of change, but I do not 

 think that it is so entirely. Here, again, confirmation must 

 come with better material. It seems probable that the darkly- 

 staining, peripheral, vitelline layer is the most recently-formed 



