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Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 513 These, like the last, are connected with the germinal epithelium, covering the outside of the ovary, by a pedicle of epithelial cells, which is nearly of the same width as the whole structure and its follicle. The germinal epithelium is in a condition of very active multiplication, the nuclei being very closely crowded together. The layers of cells which surround the central mass cannot be differentiated ; they present the appearance of a mass of cells continuous with the germinal epithelium and forming a layer of cells three or four deep ; only here and there (fig. 10, bl) were traces of the irruption of the stroma in the shape of small blood-capillaries. The cells which constitute this peripheral layer are precisely similar in their character to the cells which form the outermost of the peripheral layers in Stage I. In two instances belonging to this stage, which I have been able to study, the homogeneous darkly-staining mass produced by the solution and fusion (?) of the protoplasm of the central cells was much more in amount than in the last described stage. Fig. 11 of Plate LIII. represents the central mass of cells, which are seen to be divided up into partly or entirely isolated clumps by the formation of this homogeneous mass, which contains also free nuclei (fig. 11, re). In the third case the condition of the central cells, so far as this fused mass of protoplasmic material is concerned, was much the same as in Stage I. On the whole these facts appear to indicate that the bodies belong to a somewhat earlier stage than those just described and shown in fig. 1 of Plate LII. Their small size, the undifferentiated condition of the peripheral layers, as well as the very small amount of stroma (blood-vessels) between the ceils of these layers, appear to me to point to this conclusion. On the other hand, the greater amount of change in the central cells, i. e. the increased amount of the deeply-staining fluid substance between isolated clumps of cells, is against such a supposition, as it is evidently a further development of a process which has only just commenced in the developing structure which I have last described. This latter reason is perhaps not a very powerful argument, because it may easily be supposed that the production of the semifluid protoplasmic substance may be hastened or retarded ; the same may be said with regard to the specialization of the follicular layers, only that a specialization in the instances observed by myself goes together with increase of size of the whole body. Accordingly I am inclined to believe that the bodies displayed in fig. 9 of Plate LIII. belong to a younger stage than those illustrated in fig. 1 of Plate LII. Stage II.-The different layers composing the follicle are more differentiated, and each individual layer is now quite recognizable. Commencing from the outside, we have the secondary follicular layer, between which and the follicular layer proper is a well differentiated vascular layer, which is easily to be made out through the whole circumference ; the blood-vessels are filled with blood, and appear as round, elliptical, or elongated according to the angle of the section. The follicular layer has tbe appearance of being only P R O C ZOOL. Soc-1886, No. XXXIV. 34 |