OCR Text |
Show 520 MR. F. E. BEDDARD os THE DEVELOPMENT AND [Dec. 7, state of preservation, but in most cases the nuclei were very well preserved indeed, showing the rounded or oval form and the granular contents. This will not apply to the germinal vesicle of the ova, which were usually rather altered, showing, however, the peripheral layer of germinal spots. It does not seem likely, therefore, that the structure just described has been so altered as to render its identification impossible. As already said, the evidence of the existence in Ceratodus of the structure formed by a fusion of cells depends upon only one case, which is an early stage corresponding to that of Protopterus figured on Plate LIII. fig. 9. This is the only example that I have succeeded in finding after a careful examination of many hundred sections. Besides these, m y sections of the ovary contain a few peculiar structures, displayed in figs. 3, 21-23, which are certainly not referable to the same series as the last, and concerning the nature and homologies of which I am in great doubt. The material at m y disposal was not sufficiently well preserved to enable me to speak with certainty as to every detail of structure; and I only succeeded in finding a very few of tbe bodies in question, so that the following account is necessarily meagre. In fig. 3 of Plate LII. is represented what I believe to be the earliest stage: it consists of a spherical mass of cells bounded externally by an apparently structureless membrane, which separates them from the surrounding ovarian stroma (a) ; the cells are mainly disposed round the periphery of the sphere, the centre of which is largely occupied by spaces in which there is no trace of any fluid ; the cells are small and rounded, with a large spherical or oval nucleus ; the nucleus, but not the cell-protoplasm, is deeply stained by the reagent used (borax carmine). The cells are exactly similar to the germinal cells so far as I could see ; and the conditions I shall describe in the next stage lead me to infer that they are derived from the germinal epithelium. The second stage differs from that just described in being still continuous with the germinal epithelium; this fact would seem to point to its being an earlier stage than that just described, were it not for another difference in its structure. The body consists, like the last stage, of a mass of cells, but in the interior is a patch of granular substance, which shows a different reaction to the staining-fluid. It is hardly at all affected by the borax carmine and has a yellowish tinge. This central mass encloses here and there a few of the more peripherally-placed cells. Of the next two stages, displayed in figs. 21-23, I am uncertain which ought to be regarded as the earlier. In both the mass of cells has dwindled down to a single layer of peripherally-placed cells (b), which, as before, are separated from the stroma of the ovary by a conspicuous aud apparently structureless membrane. In the centre of the cells is a spherical or oval mass of a substance somewhat granular in appearance, which is not separated from the peripheral layer of cells by any membrane, but only by shrinkage. This mass (figs. 21 and 22) is of a yellowish tint, hardly |