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Show 1886.] OVUM OF LEPIDOSIREN. 285 much like that of the white yolk-spheres of Sauropsida. In any case these bodies are not supposed to have an extrinsic origin, but to arise within the ovum. On the other hand, a penetration of follicular cells through the micropyle (loc. cit. pi. i. fig. 6) appears really to occur in many osseous fish and to be comparable to the proliferation into the ovum of the follicular cells in Lepidosiren. Kolessnikow l confirms the accuracy of His's results as to the entrance of leucocytes into the ovum, but does not think this process to have much functional importance. In Mammals a number of observations have been published which tend to show that there is a migration of cells, which is evidently comparable to the facts which I have detailed above in Lepidosiren. Lindgren 2 has described such a migration of follicular cells, and figures an ovum which is half filled with unaltered follicular cells. Von Sehlen 3 and H. Virchow 4 have confirmed the accuracy of Lindgren's observations. More recently Schafer5 has described a remarkable series of changes in the Rabbit's ovum which do not altogether tend to the same conclusion. In young ova, which are as yet surrounded b}T a follicle consisting of only a single layer of cells, peculiar cells make their appearance in the peripheral regions, and ultimately form a single layer of cells which surround the ovum, lying beneath the follicular layer. Schafer believes that these cells are not derived from the follicular layer, but they originate in the ovum. He compares very justly his own observations with those of Kuppfer on Ascidia canina. Kuppfer6 had shown that cells appear in the interior of the ovum and range themselves round its periphery. Kuppfer, however, believed that these cells originate in the ovum itself, and are not, as Kowalevsky supposed, a product of the follicular epithelium. His statements therefore are in complete accord with those of Schafer ; while Lindgren, von Sehlen, and H. Virchow describe a process in the maturation of the mammalian ovum which is more comparable to that described by Kowalevsky in the case of the Ascidians. The latest writer on the mammalian ovum, Mr. Heape, did not find any such migration of follicular cells, and concludes that the observations put on record by Lindgren, von Sehlen, and Virchow are based upon abnormal processes. It is to be noted, however, that the type studied by Heape7 (Mole) was not studied by any one of these naturalists, and this fact may possibly account for the discrepancies in their statements. In the latest edition of Quain's 'Anatomy' it is suggested that the entrance of follicular cells into the ovum described by Lindgren, von Sehlen, and H. Virchow may be an abnormal process and not a regular 1 Archiv f. mikr. Anat. vol. xv. (1878) p. 399. 2 Arch. f. Anat. u. Phys. 1877. 3 Arch. f. Anat. u. Phys. 1882. 4 Arch. f. mikr. Anat. Bd. xxiv. (1884). 5 Proc. Roy. Soc. vol. xxx. (1880) p. 243. 6 Arch. f. mikr. Anat. Bd. viii. See also the papers of many others (Sabatier, Eoule, Fol) on the Ascidian ovum, and the facts referred to in footnote on p. 276, supra. These have been lately summed up in Quart. Journ. Micr. Sci., June 1881% by Mr. Arthur Thomson. 7 Quart. Journ. Micr. Sci., Feb. 1886. |