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Show 522 MR. F. E. BEDDARD ON THE DEVELOPMENT AND [Dec. 7, Ceratodus, is the resultant of a large number of cells the protoplasm of some of which undergoes certain changes and forms a more or less fluid mass with the original nuclei suspended in it ; this mass appears around and between the rest of the cells, which are destined for its nutrition. The whole structure is surrounded by a definite follicular layer, which also shares in its nutrition by the formation of yolk in its cells and their proliferation inwards. These bodies are surrounded by cellular layers which correspond exactly to the layers which surround the ova of other vertebrates; the difference is that instead of there being a single cell which grows at the expense of the rest, the interior of the mass is formed by numerous cells, all equivalent. The bodies may be distinguished as multicellular or plasmodial from the ordinary unicellular ova. Tbe share which the follicular epithelium takes in the nutrition of the ovum, I have discussed in detail in my former paper and need not refer to it again here, except to remark that the elaboration of food-material in the follicular layer and its absorption by the ovum has of course no relation whatever to m y view tbat the ovum is a cell-complex. Certain writers have adduced arguments of this kind as a disproof of the unicellular nature of the ovum, which to m y mind have no force. On the other hand, tbe developmental facts with respect to the cells within the follicle appear to me to be difficult to interpret otherwise than on the assumption that the ovum has the value of more than a single cell. It is true that I have been unable to detect any earlier stages than the one figured on Plate LII. fig. 1 ; but the intermediate stages between that and the mature ovum are fairly complete. The discovery of the earlier stages is of great importance; it would decide among others the very important question whether the central mass of cells is, or is not, derived from primitive ova recognizable as such in the germinal epithelium, and whether or not the central mass of cells is formed by the migration inwards of a number of these cells or by the repeated division of one. But, whatever may be the answer to these questions, I have, I think, proved that the ovum is formed out of this central mass of cells. Some of these cells are apparently used as pabulum, but others fuse together into a mass of semifluid substance, which bears a very close resemblance to the liquor folliculi of the mammalian follicle. The resemblance is still more striking if we accept Waldeyer's statements that the liquor folliculi is produced by a direct metamorphosis of the follicular cells, their nuclei remaining, as in the case of Protopterus, suspended in it. This substance, however, in Protopterus has not a mere passive function, serving, as in the mammal, to aid in the expulsion of the ripe ovum, possibly also in its nutrition; it retains the activity of the cells from which it is derived and secretes yolk ; it must therefore be looked upon rather as a plasmodium of these cells than a product of their degeneration, although its deep staining with borax carmine, as opposed to the very light staining of the remaining cells, indicates some chemical change. Furthermore, there is no evidence of any |