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Show 1889.] ON T H E ATTACHMENT OF EMBRYOS IN AURELIA. 583 5. Note on the Mode of Attachment of the Embryos to the Oral Arms of Aurelia aurita. By E D W A R D A. MINCHIN, Keble College, Oxford. [Eeceived October 31, 1889.] (Plates LVII. & LVIII.) Some little while ago, when engaged in dissecting a series of Aurelia aurita in the Morphological Laboratory at Oxford, I noticed that a great number of the specimens supplied had the oral arms covered with little knobs or swellings, which, though varying greatly in size in different specimens, were always, when present, quite visible to the naked eye. I was unable at the time to obtain any information as to the meaning of these appearances, and therefore proceeded to investigate them by cutting sections of the arms. I then found that the knobs were really little stalked capsules or pouches containing embryos of Aurelia, formed as evaginations of the wall of the groove running down the arm, and with their lumen communicating with that of the groove through the more or less narrowed stalk. This is readily seen from the annexed figures. Fig. 1, Plate LVIII., represents an oral arm covered with the brood-capsules, drawn about three times natural size. Fig. 2, Plate LVII., represents a transverse section of an oral arm which bore no brood-capsules, in order to show the structure of the arms -namely, ectoderm (ect.) externally, endoderm (end.) lining the lumen of the groove internally, and between the two mesogloea (mes.), which is very thick at the bottom of the groove. The margins of the groove are produced into numerous "digitellse" (d.), finger-like processes of the ectoderm, containing a core of mesogloea and thickly covered with nematocysts. Fig. 3, Plate LVII., represents a transverse section from an arm which bore very few, and comparatively small, brood-capsules. Two capsules are seen on the left side of the figure, one of which (a) is cut through its stalk, and the other (b) a little to one side of it. Figs. 4 and 5, Plate LVII., represent in outline two more sections from the same series through the brood-capsules a and b of figure 3, in order to show the way in which a becomes closed off from the groove (fig. 4) and b becomes bifid (fig. 5). Fig. 6, Plate LVIII., represents one side of a transverse section through an oral arm which bore numerous, and relatively very large, brood-capsules. Four of the capsules appear in the section, one of them (e) cut through the middle of its stalk, two others (c and d) j ust to one side of their respective stalks, and a fourth (/) so far from its stalk that it appears as if detached from the arm altogether. From these figures it is evident that the capsules are formed as simple evaginations of the walls of the groove of the oral arm. They are hence lined by endoderm internally and ectoderm externally, with more or less mesogloea between the two. In the smaller |
