oj^ the Fishery Board for Scotland. 



109 



alterations before it is transformed into the cytoplasm and yolk-spherules. 

 This metabolism no doubt has its centre in the germinal vesicle, and the 

 most prominent objects in the gerniinal vesicle are the nucleoli, which, 

 as we have seen, move to the periphery, i.e. nearest to the cytoplasm, 

 very shortly after cytoplasm begins to be formed. This movement of the 

 nucleoli is comparable to the migration of the germinal vesicle to the 

 seat of growth in growing plant cells. 



Some features of the germinal vesicle of the large opaque eggs were 

 also studied in fresh specimens. By graduated compression and rupture 

 of the egg-capsule it was not difficult to isolate the germinal vesicle so as 

 to be able to examine it with medium and moderately high powers, and 

 also to ascertain something concerning its physical properties by watching 

 its behaviour when escaping from the aperture, and under different 

 conditions. Some appearances of this kind are shown in Figs. 11, 12, 13, 

 PI. I. In the fresh egg of the flounder, taken from the living fish and 

 immediately examined in the ovarian fluid, the germinal vesicle on 

 extrusion appears as a clear, jelly-like, or tenacious, semi-fluid body. It 

 adheres to the outer surface of the egg-membrane, and rolls along it, but 

 clinging to it, in the streams of yolk produced by variations in the 

 pressure on the cover glass. It changes its shape or becomes indented 

 when a stream of yolk impinges against it, but in a slow, flowing fashion, 

 and also in passing between two adjacent eggs on the slide ; it is 

 obviously soft, non-elastic, and tenacious. The germinal vesicle of eggs 

 removed from a fish which has been dead for some time does not behave 

 in this way. It has lost its adhesiveness and becomes less soft, but can 

 be readily made to change its shape, and it easily ruptures (which, 

 indeed, very frequently happens as it escapes from the aperture in the 

 zona), and a clear, somewhat syrupy, fluid escapes, to which yolk- 

 spherules become attached. In the opaque eggs of the cod, haddock, 

 and plaice, which had been kept (in January and February) for a day or 

 more, the contents of the isolated germinal vesicle were much more fluid, 

 running out among the yolk-spherules and disappearing. The germinal 

 vesicle measures between one-third and one-fourth of the diameter of the 

 egg. On examination with a moderately high power, delicate fibrillse 

 could sometimes be detected passing from the surface among the yolk- 

 spherules attached to it (Fig. 18, PI. I.). The surface also shows 

 distinct but very fine and minute dotting, an appearance probably 

 due to rupture of the protoplasmic threads that pass between 

 the yolk-spherules. Eetraction of the contents allowed the membrane 

 to be seen very distinctly ; that of a cod's ovum, which contained some- 

 what watery contents, was left after the latter escaped as a delicate 

 wrinkled film. 



The addition of a 1 per cent, solution of acetic acid, or of a 10 per 

 cent, solution of common salt, clears up the yolk, and darkens and 

 renders visible the germinal vesicle ; the salt solution acts more slowly. 

 The ordinary opaque eggs left to soak in a 1 per cent, solution of 

 common salt for some hours clear up imperfectly, but sufficient to allow 

 the germinal vesicle to be seen as a translucent area in the centre. These 

 re-agents reveal the dark peri-nuclear ring of oil-globules in those eggs 

 which possess it ; but after a little the globules collect for the most part 

 towards the surface of the egg owing to the disintegration of the 

 protoplasmic strands. 



The thin cortical layer of protoplasm which everywhere encloses the 

 yolk is continuous with the interstitial protoplasm that passes between 

 the yolk-spherules, and is, as we have seen, separated from the zona 

 radiata, at least in some cases, by a delicate membrane. The cortical 



