OCR Text |
Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 509 it possible that the greater part of the tunic, which has apparently the structure of ordinary connective tissue, is in reality derived from the original germinal epithelium, a view which tallies with the fact that in some instances the cells of the tunic appear as if about to assist in forming the follicular epithelium of some of the developing ova." The nuclei of the peripheral layer of cells are much like those of the proliferating germinal epithelium, though not quite so darkly stained-perhaps for mechanical reasons. Their shape varies considerably, some being rounded and some more fusiform; but these two extremes are united by numerous intermediate conditions. For the most part the nuclei have taken up the staining-fluid unequally ; a patch at one extreme of the nucleus is more darkly coloured, and from this radiate slender threads towards the opposite extreme of the nucleus ; the interstitial part of the nucleus is stained of a pale pink. The different forms of these nuclei are represented in fig. 5. The differential staining of the nucleus undoubtedly corresponds to the differentiation of its substance ; and the star-like form of the darkly staining part suggests a connection with the phenomena of nuclear division ; but I have not observed any cases in which the centres of the star-like bodies in two adjacent nuclei were opposed; in every instance the darkly stained extremity of the nucleus was directed away from the germinal epithelium and along the axis of the layer of cells; this uniformity in the nuclei gives them the appearance of being in rapid motion, of being as it were swept along by a current round the periphery of the sphere of cells. Here and there the continuity of the layer of cells is interrupted by blood-vessels (b.l), usually of small dimensions, which are the forerunners of the richly developed vascular supply of these same bodies in later stages of development. The appearance of blood-vessels was more common on that side furthest from the germinal layer than on that nearest to it. Towards the opposite extremity of the sphere of cells, i. e. that furthest removed from the outside of the ovary, the character of the nuclei of the peripheral layer of cells becomes changed. In this region the nuclei have lost the peculiar arrangement of the nuclear substance and present the appearance of ordinary nuclei (fig. 5 a); that is to say, they are oval bodies with finely granular contents and here and there round, darkly stained particles which are disseminated throughout the nucleus. Within the peripheral layer of cells is another layer of cells which becomes ultimately comparable to a follicular epithelium, Even in this early stage of development it is for the most part distinguishable from the outer layer of cells; the distinction is not only in the character of the cells and their nuclei, but in an absolute line of demarcation which separates the two layers ; this consists (fig. 1, x) of a narrow band of structureless substance, which bears the closest possible resemblance to a substance produced by the fusion of some of the central cells of the sphere, and which will be described shortly. It is possibly formed by a metamorphosis of the peripheral layer of |