Johnston, Morphology of the Head. 259 



and ventral roots of the cranial and first spinal nerves and can 

 state their origin, relation to the myotomes and distribution. 

 Reference to Fig, 10, with which Hatschek's Fig. 11 and 

 Koltzoff's diagrams may be compared, will make the relations 

 clear. The profundus passes over the caudal and ectal surface 

 of somite i, which is innervated by N. Ill, and is distributed to 

 the ectoderm outside and in front of that somite. The trigem- 

 inus passes over the caudal and ectal surface of somite 2, in- 

 nervated by N. IV, and is distributed to the ectoderm overlying 

 that somite and its mandibular arch. N. VII passes caudal to 

 somite 3, innervated by N. VI, and supplies the hyoid arch. 

 It has no cutaneous component. N. IX lies between somites 4 

 and 5 and has a dorsal ramus passing out along this septum 

 (Hatschek's Oec. /). The vagus ganglion appears between 

 somites 5 and 6 and its dorsal ramus passes out along this sep- 

 tum. There is no ventral root to either somite 4 or 5 on account 

 of the early reduction of the mesial part of each of these somites. 

 Behind somite 6 (Koltzoff) both dorsal and ventral roots are 

 present. The ventral root of somite 6 probably innervates 

 myotomes 4, 5 and 6 in the larva. The dorsal root sends a 

 ramus to the skin of the occipital region {^Occ. j, Hatschek). 

 We should expect the dorsal rami of N. IX, X and Sp. i to be 

 distributed to the ectoderm overlying somites 4, 5 and 6 re- 

 spectively. It is probable that this is their actual position in 

 the embryo but owing to the migration forward of myotomes 

 4, 5, 6 and 7 during development myotome 5 is brought beneath 

 the ectoderm belonging to somite 4, myotome 6 beneath the 

 ectoderm of somite 5, etc. This accounts for the apparent 

 distribution of Hatschek's Occ. i, 2 and j to the area over 

 myotomes 5, 6 and 7. These dorsal nerves all persist in the 

 adult but the ventral nerve of somite 6 disappears and myo- 

 tomes 4, 5, 6 and 7 are all innervated by the ventral nerve of 

 myotome 7. From the work of these two authors, then, it 

 would appear that the relations in cyclostomes are the same 

 as in Amphioxus, the dorsal nerves innervating the ectoderm 

 overlying the myotomes next anterior to the dorsal roots. 

 Two further remarks are to be made. Neal's suggestion that 



