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 depres- 

 sion increases in depth, and this leads to the formation represented 

 by stage D. The dorsal sheath at this stage has entirely lost con- 

 tinuity with the ventral sheath, principally because its lateral por- 

 tions and the original lateral portion of the ventral layer have 

 become wholly reduced. The rest of the dorsal sheath at this level 



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 ven- 

 tral sheath alone. With the disappearance of the rest of the dorsal 

 sheath, the 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, con- 

 sisting of a right and of a left half (stage F). In some species, 

 e. g. in Nectarinia sple^idida, 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 milli- 

 metres 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, 



