SEXUAL ORGANS AND THEIR PRODUCTS IN PHORONIS. 



147 



already be invested by the ovarian tissue right up to the blind end, others 

 may exhibit it in the proximal portion only, the distal portion still retain- 

 ing the nutriment layer to a greater or less extent. Occasionally there 

 may exist even such as Lave just begun to bear nutriment cells and 

 therefore remain quite thin in the distal part. With full development of 

 the ovary, all the capillaries concerned in its formation should have the 

 nutriment layer entirely replaced by the egg-follicles. 



To follow the growth of the oocytes, it is at first generally spherical 

 in shape, with the vesicular and relatively large nucleus situated in the 

 centre. As it grows it soon assumes an ovoid or flask-like shape, being 

 attached to the capillary by the narrower end. This is doubtless due to 

 mutual pressure exercised by the oocytes at base, as they find attachment 

 close together on the narrow axially running capillary (see fig. 4). The 

 nucleus is now seen to occupy an eccentric position in tin broader end 

 of the egg-body. The follicular envelope is visible for some time as a 

 flat epithelime showing compressed nuclei at rather long intervals ; but 

 as the egg approaches full size it becomes so distended and thinned out 

 that it appears as a fine structureless membrane, in which the nuclei can 

 no longer be made out. In small oocytes, such as are represented in fig. 

 2, the vitellus is finely granular and is deeply stained by basic stains, such 

 as hcematoxylin, gentiana-violet, etc. In more advanced stages, it is 

 somewhat coarsely granular and then greedily takes up acid stains, such 

 as eosin, Congo-red, orange-G, etc. instead of basic. 



The large clear nucleus exhibits a net-work of linin ; it is wide- 

 meshed though densely arranged in immediate environment of one of the 

 nucleoli, of which there can be distinguished two (see fig. 4, E) Both 

 nucleoli are spherical and very distinct, their size growing in proportion 

 with the growth of the oocytes. The one nucleolus (u) is distinguished 

 by the fact that it consists of two parts : a small sphere which is but 

 very lightly stained by nuclear stains, and another larger sphere which is 

 deeply stainable. At first, while the oocyte is still very small, the former 

 lies simply apposed on the surface, but later sinks completely into the 

 substance, of the latter. In the stages shown in fig. 4, the nucleolus in 



