OVAEIAN OVUM OF MAESUPIALIA AND MONOTEEMATA. 121 



whole of ovarian development. In one instance I calculated 

 that it was '0013 mm. in thickness, and, even allowing for a 

 considerable error, the result is very remarkable. The ovum 

 itself (as prepared by the method above described) appeared as 

 a very delicate network, which did not stain with reagents. 

 The nucleus (germinal vesicle) was enclosed in a distinct 

 limiting membrane, and appeared in two forms distinguished 

 by their behaviour with staining fluids. In the one case it 

 stained deeply (see fig. 3), and in the other not at all (see 

 fig. 2). It generally showed a coarse network, and it always 

 contained a distinct spherical dark-coloured nucleolus (ger- 

 minal spot). I could never distinguish the nucleus in the 

 ovum of a large follicle. In the smallest ova the nucleus ap- 

 peared to be always central, and it became eccentric during 

 increase in size (compare figs. 4, g,f, with 2 and 3). The first 

 traces of a cavity in the granulosa resemble the corresponding 

 change in higher animals, and at first the clot formed in the 

 cavity (by the reagents) appears to be identical with the 

 coagulated liquor foUiculi. The former shrinks away from the 

 walls of the cavity, remaining attached by threads, and in 

 structure it is homogenous or finely granular. While the clot 

 is thus normal the ovum is still embedded in the thickened 

 granulosa cells, which at this stage may be called a cumulus 

 proligerus. At this point the only essential characteristic of 

 the follicle is the absence of radiating columnar cells round 

 the ovum (see fig. 4). Duriug further development the fol- 

 licle increases immensely in size, and the granulosa cells form 

 the abundant follicular contents so rapidly that (as the follicle 

 widens) the layer is much reduced iu thickness. The cells 

 round the ovum undergo a similar change, and thus the ovum 

 comes to be isolated in the follicular contents, surrounded by 

 vanishing layers of cells (see fig. 5). The change continues 

 until in the large follicle the ovum is left embedded in the 

 central substance, completely detached from the membrana 

 granulosa, and only bounded by its zona pellacida (see fig. 6). 

 Nevertheless, there are slight thickenings on the granulosa at 

 the point nearest to the ovum, probably due to the fact that 



