Johnston, Morphology of the Head. 187 



disappears, that of somite 5 the more slowly. The ganglion 

 of the glossopharyngeus crowds outward, downward and for- 

 ward between the mesial and lateral parts of somite 4, while 

 the vagus ganglion assumes the same relation to somite 5. The 

 lateral part of somite 4 sends dorsal and ventral buds forward 

 above and below the ear. Somite 5 sends a dorsal bud above the 

 ear and similar buds follow from somite 6 and perhaps somite 7. 

 These dorsal buds of somites 4, 5, 6 (and 7) form the continu- 

 ation of the body muscles on the top of the head as far for- 

 ward as the olfactory pit. The great extent anteriorly of this 

 musculature in Petromyzon is connected \\\X.\\ the buccal appa- 

 ratus. Such dorso-lateral portions are found only in somewhat 

 more caudal somites in selachians: somite 7 in Acanthias (58), 

 and Pristiurus (119); somite 8 in Spinax(i2) somite 12 in Tor- 

 pedo (112). The absence of this lateral part in the region of 

 the cranial nerves accounts (Sewertzoff, Koltzoff) for the 

 position of these nerves outside the myotomes. In Petromy- 

 zon the lateral portions of the myotomes cover the cranial gang- 

 lia just as the trunk myotomes cover the spinal ganglia in all 

 vertebrates. P'rom this it seems that no somite completely dis- 

 appears in Petromyzon but the myotomes of the adult repre- 

 sent the full quota of muscle segments. 



h. Some of the myotomes in the caudal part of the head 

 region send ventral muscle buds down behind the gills and for- 

 ward ventrally to form the subbranchial musculature. In sela- 

 chians this begins with myotome 8 or 9, in Petromyzon with 

 myotome 10. It seems that the number of gills determines 

 which myotome shall be the first to contribute to these mus- 

 cles. As the number of gills decreases more anterior myo- 

 tomes are able to send buds into the subbranchial region. 



z. Each of the myotomes is supplied by a true ventral 

 nerve root. The number of these roots in the postauditory 

 region of the adult depends on the number of persistent myo- 

 tomes, and in the embryo transient roots appear corresponding 

 to transient myotomes. A more detailed treatment of this re- 

 lation will be given farther on (Sec. 12). 



j. There appears in all gnathostomes a variable amount of 



