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Show 64 DR. H. GADOW ON THE SUCTORIAL [Feb. 20, thus becoming convex whilst the dorsal layer becomes slightly concave. A little further forward, we observe that the ventral sheath near its upper or lateral end sends off a small outgrowth, which, first directed upwards and inwards towards the middle line, soon curls its free and slightly thickened end up, with its concave side looking downwards and inwards (stages B and C, fig. 1, p. 64). At the same time the dorsal half of the horny sheath becomes thinner and gradually loses its connexion with the lateral edge of the ventral sheath, whilst its median portion forms a depression. This depression increases in depth, and this leads to the formation represented by stage D. The dorsal sheath at this stage has entirely lost continuity with the ventral sheath, principally because its lateral portions and the original lateral portion of the ventral layer have become wholly reduced. The rest of the dorsal sheath at this level Fig. 1. Sections of the tongue of Cinnyris. forms now a deep canal or semi-tube with its open part looking upwards, forming a deep longitudinal furrow on the dorsal side of the tongue. A little more in front, near the tip of the os entoglossum, the dorsal sheath, owing to the tapering pointed shape of this part of the tongue, is reduced to a small bit of only half horny epithelium, situated in the middle line on the dorsal surface of the ventral sheath ; and the whole sheath of the tongue is now represented by the ventral sheath alone. With the disappearance of the rest of the dorsal sheath, tbe free edges of the ventral one curl more and more inwards, so as to meet in the middle line and to form a complete tube, consisting of a right and of a left half (stage F). In some species, e. g. in Nectarinia splendida, the free edges continue to curl inwards until this leads to the formation of a median and two lateral tubes, which three tubes, of course, are continuous with each other, since they are simple modifications of the primary tube. After the disappearance of the rest of the dorsal sheath, we notice a slight swelling of the median portion of the tube ; and a few millimetres in front of this little ridge the tube separates or splits up into a right and into a left half. The median edges are very thin, |
