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Show 518 MR. F. E. BKDDAUD ON THE DEVELOPMENT AND [D eC. 7 migrated into the interior ; the principal evidence is in favour latter supposition. The central mass itself is, as already stated, mainly occupied by a mass of yolk-spherules; these are deeply stained by the colouring-reagent. Among the mass of yolk-spherules are numerous cells which are more scattered in later stages; many of them are in course of degeneration, as evinced by the characters of the nuclei. In m y former paper I have figured (loc. cit. pi. xxix. figs. 9-20) a number of such cells, and need not refer to the matter again here. In no case could I observe the faintest trace of a germinal vesicle, nor does any one of the cells found in the interior of the body show any preponderance in size, or difference of any kind from the rest. The vascular layer is highly developed in this stage, in accord with the rapid growth of the follicular cells. Outside the vascular layer is the secondary follicle-layer, which has much dwindled in importance, and has a still closer resemblance to stroma than it had in the earlier stages. CONTENTS OF THE OVARY OF CERATODUS. The ovary of Ceratodus, like that of Protopterus, contains, besides the ordinary ova which follow a normal course of development, other remarkable structures similar to those of Protopterus. The normal kind of ova, which are by far the most abundant, represent a single cell, and agree in most details of structure with the ova of Amphibia and the corresponding ova in Protopterus. The mature ova are filled with rounded yolk-spherules approximately of equal size, and entirely unstained by prolonged immersion in borax carmine ; the periphery of the ovum is occupied by a delicate layer of egg-protoplasm in which pigment-granules are imbedded ; the egg is covered by only a single membrane, which is moderately thick with radial pores; the follicular epithelium is a single layer of flattened cells, of which the nuclei alone are obvious in m y preparations. Some ova belonging to this stage are figured by Ayers in his paper. In rather younger ova there is a ball of protoplasm in the centre surrounding the germinal vesicle, and not yet invaded by the formation of yolk; the germinal vesicle has a peripheral row of germinal spots as in Protopterus. The formation of the yolk appears also to be on the whole very similar to the process described in Protopterus. Aggregations of yolk-granules make their appearance throughout the ovum, but do not seem to be confined at first to the peripheral layers. In the younger stages the yolk-particles are deeply stained by borax carmine, but not in the adult ova. I have already stated in the remarks introductory to this paper that, as regards my specimens, Ceratodus differs from Protopterus in the extreme rarity of those bodies which are apparently formed by a fusion of a number of distinct cells. So very rare are these structures that after a diligent search I only succeeded in discovering a single case which could be in all probability referred to Stage I. in the developmental history of Protopterus (see p. 508). Certain other |
