ORIGIN AND STRUCTURE OF THE HEAD 61 



that they can no longer be distinguished from each other. 

 From the exact place, however, where the lateral placode of the 

 N. facialis was formerly found, spring the sensory branches 

 of the facialis, the ramus ophthalmicus superjicialis, the 

 ramus buccalis and mandibularis. These innervate the cranial 

 part of the lateral line system and, according to Klinkhardt, 

 grow out from the lateral placode of the latter in the same 

 way as the ramus lateralis of the vagus, i.e. within the 

 ectoderm, intraepithelially. 



The obvious inference, therefore, is, that in these ganglia 

 of the lateral sense-organs we have to look for the lateral 

 or parapodial ganglia of Annelids, which accordingly have 

 been lost in the trunk and preserved only in the segmented 

 region of the head, which in more than one respect exhibits 

 primitive features. The advantages of this conception over 

 that of EISIG will be evident. Just as with the ganglia of the 

 lateral sense-organs in Annelids, the dermatogeneiic ganglia 

 of the Vertebrate head arise in close connection with 

 epidermal sense-organs, while the spinal ganglia have 

 no primary relation whatever to them. The segmental 

 communications of the spinal ganglia with the lateral sense- 

 organs have only been preserved in the head, in the trunk 

 they have been replaced by the collector. To this conception 

 the presence of a segmental connection of the spinal ganglia, 

 with the corresponding organs of the lateral line, as described 

 in Petromyzon by JULIN (1887) and ALCOCK (1899), but denied 

 by DOHRN (1888. and FuRBRINGER (1897), would no doubt 

 be a valuable support. Similar connections were found by 

 HOFFAUNN (1901, p. 39, 45) in Urodelans and described and 

 illustrated by him in a very positive way as regular anasto- 

 moses of the dorsal branch of the S**" — 20**' spinal nerves 

 with the ramus lateralis vagi. 



As to the epibranchial fusion of the dorsal cranial nerves 

 with the epidermis, I can only suppose with BEARD (1885) 

 that it has arisen in connection with the gill-slits to which 

 it shows such close relations. PRORIEP (1891, p. 63) has- 

 pointed to the close connection between the epibranchial 

 ganglia and the rudiment of the thymus, a connection which, 

 however, gets lost in the adult state. In no case have these 

 branchial sense-organs been found to persist in the adult.. 



Different theories on the metameric structure of the head. — 

 As already observed, the theory advanced here appears to 



