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Show 672 PROF. G. B. HOWES ON THE VISCERAL [Dec. 2, may have actually caused the absorption of its substance ; or whether the latter may have not been induced by other means, the glands named having merely accommodated themselves to the exigencies of the case. The spleen had been unfortunately removed, to a large extent, in m y specimen before it reached m y hands (cf. Plate LVII. fig. 3, s.) ; sufficient, however, remains to show the presence of a couple of well-marked furrows. One of these (f.p-) indicates the point of apposition with the head of the pyloric gastric sac (py.) ; the other (f.m.) that of strangulation or embrace by the mesentery. That portion of the spleen which lay to the right of the latter (= that marked s. in figs. 2 & 3) had grown out into a veritable hernia. In view of the very variable extent to which this organ may force its way between the folds of the mesentery among Plagiostomi in general, I am inclined to adopt the second of the two alternatives postulated above, and to regard the absorption as perhaps not primarily due to this hernia-like extension of the glandular structures named. The spleen of the Batoidei is remarkable among that of all vertebrates for its enormous development. It lies in the bay formed between the cardiac and pyloric sacs of the stomach, and projects freely to the right side. It either extends under cover of the stomach and intestine, giving rise to a solid mass which lies immediately beneath the backbone, and not unfrequently fills the interspace between the genital glands ; or it embraces the left side of the valved segment of the gut (ex. Rhinobatus and Trygonorhina), in a manner somewhat resembling that in which the embryonic supra-renal body of mammals "caps" its corresponding kidney. It will be observed that in Hypnos the spleen (s., fig. 2) passes behind the main trunk of the (anterior) superior mesenteric artery (a.m.1). In many Plagiostomes, it shows a marked tendency to extend either in front of the same or between its branches; consequently, while the facts seen in the absorption of the mesentery of Hgpnos beyond doubt furnish the clue to the rationale of this process as it applies to the living Chondrichthyes generally, they would appear to denote the initial phase in one of a possible series of variations in the same. The absorption of the mesentery is a phenomenon which has long been recognized among other vertebrated animals, and consideration of the facts concerning it yields an interesting result. Rathke has long ago described it1 in tbe Turbot and Gar-Pike, Owen in the Pipe-fish 2, while both these observers have recorded it for the Cyprinidae. While in the Myxinoids the mesentery is continuous, in the Petromyzontidse it is absorbed to the maximum degree-persisting, as is well known, at the extreme anterior and posterior ends of that portion of the gut which lies within the post-pericardiac coelom, 1 ' Ueb. den Damkanalen und Zeugungsorgane d. Fische.' Halle, 1824 pp.104-105. 2 Comp. Anat. & Phys. vol. i. p. 424. Cf. also Cuvier and Valenciennes, Hist. Nat. des Poissons, vol. i. p. 507. |