PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 621 



rudiment of a muscle which will unite with the muscle already formed as just described 

 on the ventral and lateral sides of the pharynx to complete the constrictor pharyngis. 

 It will form the " dorsal part" of this muscle of Wiedersheim. 



Plate fig. 2 shows this outgrowth from y, and also the extreme front end of the 

 ventral portion of the constrictor. In this section, although it is slightly behind the 

 position of the section shown in fig. 1 , we have the posterior portion of the pericardium 

 cut, owing to this having moved backwards since stage 30. 



Thus the " dorsal part " of Wiedersheim is somatic in origin, the ventral part 

 splanchnic. 



Fig. 3 shows the condition of the muscle rudiments at stage 34. 



The two constituents have coalesced to form a complete sphincter muscle. The 

 limits of the two components are marked by the connective tissue intersections. 



Fig. 5. — Protoptenis, 5*5 cm. transverse section through the occipital region, showing the 

 relations of the dorsal and ventral parts of the constrictor pharyngis. ao., aorta; 

 c.ph.d., dorsal part of "constrictor"; c.ph.v., ventral part of constrictor; gl. cart., 

 glottis cartilage ; occ.arch., occipital arch ; ph., pharynx ; ram.int., ramus intestinalis 

 of vagus. 



At this stage and onwards the occipital nerve y can be distinctly traced into 

 this muscle. 



At present this muscle seems to act as a simple constrictor, as named by Wieders- 

 heim (cf. Plate fig. 3). But later on, the ventral, splanchnic portion attaches itself 

 to the auditory capsules, occipital ribs, and axial skeleton, and so is able to act as a 

 constrictor by itself, by compressing the pharynx between itself and the notochord 

 (text figure 5). As described by Wiedersheim, a part of this becomes differentiated 

 as the dilatator of the glottis — (posterior to the section in text figure 5). The dorsal, 

 somatic portion of the muscle becomes attached to the occipital rib, and in the 

 middle line to the dorsal wall of the pharynx, and so appears to act as a dilatator 

 pharyngis by raising the dorsal wall of the otherwise dorso-ventrally compressed, slit- 

 like cavity of this part of the alimentary canal (see the figure). 



