Johnston, MorpJiology of the Head. 225 



As noticed above, the medial parts of somites 4 and 5 disappear 

 without forming muscle, the lateral parts form muscles on the 

 dorsal side of the head. All following somites persist as myo- 

 tomes. The somatic motor nerves of somites 4 and 5 are not 

 found. The nerve of somite 6 seems to innervate somites 4 and 

 5 for a time, then disappears, and somites 4, 5 and 6 are in- 

 nervated in the adult by the nerve of somite 7. Following this 

 each myotome has its ventral nerve. 



Following the ear there is in the embryo one dorsal nerve 

 root or ganglionic anlage between each two somites, corres- 

 ponding to Nn. IX, X, Sp. I, 2, etc., and according to Hatschek 

 (44) each of these is represented in the adult by a dorsal ramus. 

 Now since the vagus root contains both the sensory and motor 

 visceral components for all the following branchial segments 

 and since the dorsal ramus is typically general cutaneous 

 in vertebrates, it seems clear that there is present in each seg- 

 ment behind the ear in Petromyzon a general cutaneous root, 

 the series continuing without interruption into the trunk. 



The glossopharyngeus is independent in Petromyzon and 

 contains (68) general cutaneous, splanchnic sensory and splanch- 

 nic motor components which form the typical rami for the seg- 

 ment containing somite 4 and the first branchial arch, ramus 

 dorsalis, r. posttrematicus, r. praetrematicus, and r. pharyngeus. 

 The vagus is treated by Koltzoff as constituted of a full 

 branchial nerve similar to N. IX, innervating somite 5 and br. 

 arch 2, and of a ramus branchio-intestinalis. This later consists 

 of a series of ganglia, one over each gill slit, connected by a 

 longitudinal nerve trunk. P'rom each ganglion arise the three 

 rami which innervate a branchial segment in the typical manner. 

 Each ganglion is formed at least in part at the expense of an 

 epibranchial placode. The nerve trunk is continued backward 

 as the ramus intestinalis. 



Koltzoff concludes from his study of Petromyzon that the 

 vagus nerve with its ramus branchio-intestinalis is not a com- 

 pound nerve formed by the fusion of two or more segmental 

 branchial nerves, but is a collector which has taken up the ven- 

 tral rami of several segrments. In this he seems to the writer to 



