STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARY. 413 



than of fibres. The fluid contents of the nucleus remain as a 

 rule, even in the hardened specimens, perfectly clear, though 

 they become in some instances slightly granular. There are usually 

 two, three, or more nucleoli generally situated, as described by 

 Eimer, close to the membrane of the vesicle, the largest of which 

 may measure as much as 0006 mm. They are highly refract- 

 ing bodies, containing in most instances a vacuole, and very fre- 

 quently a smaller spherical body of a similar nature to 

 themselves.i Granules are sometimes also present in the 

 germinal vesicle, but are probably only extremely minute 

 nucleoli. 



In ova of 0*5 mm. the germinal vesicle has a diameter of 0"12 

 mm. (Fl. XVill, fig. 21). It is usually shrunk in hardened i^ 

 specimens though nearly spherical in the living ovum. Its con- 

 tents are rendered granular by reagents though quite clear wdien 

 fresh, and the reticulum of the earlier stages is sometimes with 

 difficulty to be made out, though in other instances fairly 

 clear. In all cases the fibres composing it are very granular. 

 The membrane is thick. Peculiar highly refracting nucleoli, 

 usually enclosing a large vacuole, are present in considerable 

 numbers, and are either arranged in a circle round the periphery, 

 or sometimes aggregated towards one side of the vesicle ; and in 

 addition, numerous deeply staining smaller granular aggrega- 

 tions, probably belonging to the same category as the nucleoli 

 (from which in the livuig ovum they can only be distinguished by 

 their size), are scattered cluse to the inner side of the membrane 

 over the whole or only a part of the surface of the germinal 

 vesicle. In a fair number of instances bodies like that figured 

 on PI. XIX, fig, 27, are to be found in the germinal vesicle. 

 They appear to be nucleoli in which a number of smaller nucleoli 

 are originating by a process of endogenous growth, analogous 

 perhaps to endogenous cell-formation. The nucleoli thus formed 

 are, no doubt, destined to become free. The above mode of in- 

 crease for the nucleoli appears to be exceptional. The ordinary 

 mode is, no doubt, that by simple division into two, as was long 

 ago shewn by Auerbach. 



Of the later stages of the germinal vesicle and its final fate, I 

 can give no account beyond the very fragmentary statements 

 which have already appeared in my monograph on Elasmobranch 

 Pishes. 



Formation of fresJt ova and ovarian nests in the post emhry- 

 onic stages. — Ludwig,^ was the first to describe the formation of 



* Compare, with references to several poiiits, the germinal vesicle at this 

 stage with germinal vesicle of the frog's ovum figured by 0. Hertwig, 

 'Morphologische Jahrbucb,' vol. iii, pi. iv., fig. 1. 



^ Loc. cit. 



