SKULL AND VISCERAL AECHES IN LEPIDOSIREN AND PROTOPTERUS. 53 



The general structure of the skull at stage 34 is shown in figs. 8 and 14, Plates II. 

 and III., from reconstructions from horizontal and sagittal sections respectively. It will 

 be convenient to begin the description from the hind end. 



The occipital arch is still an obvious neural arch, only distinguished from the 

 succeeding ones by its greater size. There is a wide gap between this arch and the 

 next one (first neural arch proper). Two nerves leave the spinal column through this 

 space, marking the loss of an arch between them. In no stage in Lepidosiren could I 

 find a trace of this arch, but it is present in a more or less vestigial condition in 

 Protopterus (fig. 15). In Ceratodus this arch is present throughout life, but remains un- 

 ossified (K. Furbringer). The two nerves mentioned are a and b of M. Furbringer.* 

 As a matter of fact, in Lepidosiren the posterior limit of the skull is sharply defined 

 by the gap between the occipital and the first true neural arches, and this boundary is 

 also clearly marked in the adult, so that unless the parasphenoid determines the back- 

 ward extent of the skull, the nerves should perhaps be called 1 and 2. These nerves 

 in Protopterus become included in the occipital region of the adult (Furbringer, and 

 cf. fig. 15 for a). 



The occipital plate has grown forward from the base of each arch along the side of 

 the notochord towards the Balkenplatte, with which, however, it has not yet fused. 

 The occipital plates of the two sides are as yet unconnected dorsally or ventrally. 

 Between the occipital arch and the vagus the occipital nerves leave the skull. Of these 

 there are usually two (y, z), sometimes three (x, y, z) in Protopterus. In no stage of 

 Lepidosiren have I found the nerve x, nevertheless at this stage (34) in both genera 

 the myomeres corresponding with the three nerves are present (fig. 8). I have desig- 

 nated the myomeres X, Y, Z, etc., in correspondence with the neuromeres x, y, z, etc. 

 The occipital arch is thus in the septum between the third and fourth metotic myo- 

 meres. I hope to return to these nerves and myomeres in a subsequent paper. No 

 traces of those arches in front of the occipital arch, whose loss is indicated by the 

 myomeres X, Y, Z, could be discovered. Immigration of cartilage cells into the chordal 

 sheath, which takes place at a later stage, also proceeded irregularly without any 

 indication of metameric divisions.! 



Sewertzoff finds that in the second stage of Ceratodus figured by him the occiptal 

 arch is in the septum between the fifth and sixth myomeres. He supposes that these 

 myomeres are Y and Z, but K. Furbringer finds an additional nerve in front of that 

 identified by Sewertzoff as x, and from other considerations also proves beyond doubt 

 that this arch separates the myomeres Z and A. Thus in the earliest stage in which 

 the occipital arch is present, Ceratodus possesses five myomeres in front of this arch 

 (V, W, X, Y, Z), and the Dipneumona in the same stage only three (X, Y, Z). At the 

 stage in question, even in Ceratodus, V and W have begun to degenerate. 



The portions of the myomeres X, Y, Z, shown in fig. 8, lie close alongside the 



* Ueber die Spino-occipitalen Nerven der Selacliien, etc., Fest.fiir Gegenbaur, 1897. 



t In Lepidosiren. In Protopterus I had no stage showing the process of this immigration. 



