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Show 524 MR. F. E. BEDDAKD ON THE DEVELOPMENT AND [Dec. 7, filled with yolk-particles at the expense of their protoplasm must tend to lose their activity for movement, their energy being spent in the elaboration of yolk ; again, the thick layer of cells surrounding the central cells would prevent any of the central cells from leaving the interior of the follicle ; the result of further growth would therefore necessarily lead either to the development of a number of distinct ova remaining permanently within the follicle, or to the excessive development of one of the cells, which would ultimately form the ovum, or, finally, to the formation of a single ovum out of the whole mass of cells. There are no facts which point to the truth of either of the first two alternatives, while all the facts at m y disposal appear to prove the third alternative ; accordingly the temporary fusion of the primitive ova in the Elasmobranch nest and the degeneration of some of them becomes permanent in the Dipnoi, the ovum being the equivalent of a whole " nest." Both Palaeontology and Anatomy point to the great age of the Dipnoi, which may therefore easily be supposed to have retained ancient characters in the structure of the ova, as they undoubtedly have in the structure of the genital ducts. It is more generally believed that the Elasmobranchs are at a still lower level of organization ; if, however, as Mr. Howes has pointed out to me, the Chimseroids are the ancestors both of Elasmobranchs and Dipnoi, it may as easily be supposed that the egg-nest of the former has been derived from the egg-nest of the Dipnoi, as that the converse process has taken place. In this case the temporary fusion of primitive ova in the Sharks and Rays is a reminiscence of their permanent fusion in Protopterus and Ceratodus. It does not seem to me possible at present to say which of these views is correct; nor indeed can any comparison at all of the two structures have any great weight until the structure of the ovary has been thoroughly examined in such types as Chimara and some of the more primitive Sharks. On the whole it appears to me possible to regard these remarkable structures in the Dipnoi as corresponding to the egg-nests of other Vertebrates ; but the apparent absence of any protoplasm in the yolk-mass renders it extremely unlikely that the structure develops into an embryo1; on the other hand it is often very difficult, in an ovum full of yolk, to distinguish the protoplasmic matrix; it is probable, however, that these structures do not undergo any further 1 I observed several ova undergoing degeneration-in one case belonging to the type discussed here. The follicular epithelium was in a condition of active degeneration, tbe cells becoming detached and passing into the interior of tbe ovum. (This process is not to be confounded with the nutrition of tbe ovum by the follicular cells recorded in this paper and in m y last; in the latter case the follicular cells are large, crammed with yolk-particles, and remain a continuous layer; in the degenerating ovum the follicular cells have decreased in size, contain little yolk, and great gaps are left by tbe disappearance of the cells.) The yolk has also undergone great changes, the yolk-spherules exhibit a vacuolated appearance and are of more irregular size, as if a number had become converted into fat-drops and bad run together; the amount of yolk also was less, and the ovum in consequence was collapsed and of irregular shape ; at several points the walls of the ovum were altogether indistinguishable. The way in which the ovum degenerates does not in fact |
