The Development of the Cartilaginous Skull ete. in Necturus. 449 
demonstrate that Stöhr (’82) was right in regarding this arch as 
homodynamous with the neural arches of the trunk. 
Having thus acquainted my reader with the early relations of 
the postotic somites in Necturus, before describing the development 
of the musculature connected with the first two spinal nerves, I 
pause to compare the postotic divisions of the mesoderm in Necturus 
with those described by SEWERTZOFF (95) in Siredon. 
In fig. 27, pl. XVII, I have copied from SEWERTZOFF’S fig. 12, 
pl. V, a horizontal section through the first postotic somites in Sire- 
don at the earliest stage which SEWERTZOFF describes. If this 
figure (27) be compared with figs. 20, 21, and 22, although the sec- 
tions are respectively horizontal and sagittal, it will not be found 
difficult to homologize somite with somite. M, the first postotie somite 
in Siredon corresponds with the mesodermie division marked 1 (5) in 
Necturus, and M, with 2(6) etc. I miss, however, from SEWERTZOFF'S 
figure, cells from the glossopharyngeal ganglion. Since the neural crest 
develops from before backwards, the Anlage of the glossopharyn- 
seus is slightly earlier than that of the vagus, and consequently 
longitudinal sections passing through the vagus Anlage (fig. 27, G.n.vg) 
must of necessity cut that of the glossopharyngeus also, the cells 
of which should lie external to M,, as the vagus Anlage lies ex- 
ternal to Mb». 
In describing these segments, SEWERTZOFF (page 60) says, that 
the »vordere Segment (M,) liegt unmittelbar hinter der Gehörblase, 
sich ein wenig unter dieselbe erstreckend (was man an sagittalen 
Schnitten dieses Stadiums sieht)<. Again (page 61), »the anterior 
segment lies vor dem Vagusauswuchs, zwischen ihm und der Gehör- 
BiaBead. Jarl. DU Der dorsale Theil des vorderen Myotoms fängt bald 
an zu verkümmern, indem er in zerstreute mesodermale Zellen zer- 
fällt, so dass in den Stadien, wenn die spinalen Wurzeln sich an- 
legen, seine Reste sich nur an den Seiten der Chorda erhalten..... 
Hinter derselben, in der dorsalen Region des Embryos, liegt das 
zweite Myotom, welches nach außen von der Wurzel des N. vagus 
liegt<. On page 62, SEWERTZOFF speaks of »das rudimentäre 
1-te Myotom, in dessen Fasern ebenfalls eine Querstreifung erschienen 
ist, seine Reste liegen zwischen der Chorda und der Gehörkapsel 
der entsprechenden Körperseite«. 
Now, according to NEAL (97), in Petromyzon the first postotie 
somite, occupying the position of M,, fig. 27, gives rise to muscle 
fibres. In the Selachii, however, no fibres develop in this somite 
Morpholog. Jahrbuch. 25. 30 
