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Show 1886.] STRUCTURE OF THE OVUM IN T H E DIPNOI. 524 affected by the borax carmine, and is throughout of a ; there is no structure resembling a nucleus to be seen. This central mass is clearly a further development of the condition described in the second stage. In one example, displayed in fig. 23, there is a difference from the condition just described in the presence of a few cells imbedded in the central mass, but clearly distinguishable from it by the nuclei being deeply stained. The cell-protoplasm was, however, hardly distinguishable from the surrounding mass. In the other example (fig. 21) the central mass contained no such cells. With the exception that it possesses no nucleus, this structure resembles very closely Platner's figure of the Gasteropod ovum ', which contains a number of cells (" Nahrzellen") within its substance, derived from similar cells lying around the ovum. The first two stages described are about equal in size; the latter two are also about equal to each other, but considerably larger (twice the size) of the former. The comparison of sizes quite supports my identification of the latter two as the later stages in development; and there can be no doubt, I think, that they all belong to the same series. These structures obviously bear a certain resemblance to the multicellular bodies in this fish and in Protopterus; and if I had not succeeded in finding in Ceratodus anotber structure undoubtedly corresponding to the multicellular body of Protopterus, I should have certainly regarded the structures at present under discussion as the representatives of the latter. The principal difference appears to be the non-formation of any secondary follicle-layer, the absence of any special vascular supply, and the fact that yolk is not formed in the early stages. With regard, however, to the apparent absence of the extra-follicular layer, it must be remembered that the close resemblance of the layer to a layer of stroma-cells has already been dwelt upon (p. 508). It seems to me very possible that the structure just described is formed by the fusion of the protoplasm of the centrally-placed cells, the nuclei themselves gradually disappearing. An examination of better material must, however, settle the question. General Conclusions, and Comparison of Ova with those of other Vertebrates and Invertebrates. The general conclusions to be drawn from the facts, in so far as they refer to the Dipnoi, have been partly summed up in the resume at the end of m y former paper on this subject. It may be taken as a proved fact that the ovary in the Dipnoi contains two kinds of structures developed from the germinal epithelium. The first kind is an ovum, the equivalent of a single cell, and is similar in all essentials to the cvum in the Amphibia. The second structure, which is very commonly met with in Protopterus and but rarely in 1 " Zur Bilduug d. Gescblechtspioducten bei den Pulmonaten," Arch. f. rnifcr. Anat. Bd. xxvi. (1886) p. 599. |