OCR Text |
Show 278 MR. F. E. BEDDARD ON THE [May 4, In older ova the germinal vesicle has altered somewhat from the characters which it presents in the earlier stages of the maturation of the ovum. In Plate XXIX. fig. 1, I have sketched a germinal vesicle precisely as it appeared in transverse section. It will be noticed, in the first place, that the germinal vesicle is very sharply marked off from the surrounding substance of the ovum, and in the section figured has become separated from it throughout a considerable area ; this separation is no doubt due to shrinkage, but serves to show the very trenchant line of division which separates germinal vesicle and ovum, since no yolk-spherules were found to remain adherent to the germinal vesicle. The substance of the germinal vesicle in ova of this stage of development has become specialized into an outer peripheral portion and a central core. These two areas of the germinal vesicle are almost as sharply marked off from each other as is the outer layer of the germinal vesicle from the surrounding egg-protoplasm and yolk. This is clearly shown in the same figure (Plate X X I X . fig. 1), and, as in the first case, is probably due to shrinking of the germinal vesicle when plunged into the preservative reagent. Along the upper side of the preparation illustrated in the figure already referred to, the central core of the germinal vesicle has shrunk away from the external sheath ; along the lower margin of the germinal vesicle the contraction has not caused a separation of the two constituents, but a split in the outer layer, which I have not indicated in the figure. The outer coat of the germinal vesicle is very easily distinguishable from the inner core; it is much more finely granular in structure, appearing occasionally almost homogeneous, and has become stained a pale pink. The inner core is more coarsely granular, and has a yellowish tinge, hardly owing to the non-penetration of the staining reagent, since the imbedded nucleoli were deeply stained. The substance of the internal core contained elliptical spaces here and there, which are possibly themselves due to contraction. The boundary line between the central core and the peripheral layer is also marked by the nucleoli, which are for the most part situated in a single layer at the extreme periphery of the central core, A few, however, as indicated in the figure, are to be found in the more central regions of the core. The nucleoli were extremely conspicuous owing to their vivid staining with the reagent, and appeared to be solid, being uniformly stained throughout. In the earlier stages of the germinal vesicle mention has already been made of the annular appearance of the germinal spots. The germinal spots were very unevenly distributed and of varying size, but were never found in the peripheral layer of the germinal vesicle. In Triton, Fwakawa has described what appear to be very similar changes in the germinal vesicle, though there is some little difference in detail; as the egg approaches maturity the germinal spots move in towards the centre of the germinal vesicle, leaving a homogeneous, more deeply stained peripheral area. |