412 



dorsal roots co-exist in the larva of Petromyzon both in the body and 

 the head regions, the internal rami of the cranial nerves having been 

 called by von Kupffer "spinalartige", and they have been observed 

 likewise by Dohrn in the vagus region of Selachii. The internal rami 

 may develop ganglia as well as the outer, but ganglia homologous 

 with those of the spinal nerves. The mixed dorsal cranial nerves are 

 homologous with the dorsal roots of Amphioxus, as His also pointed 

 out, in which form and in Petromyzon the presegmental origin of the 

 nerves may be demonstrated. 



It is possible with the facts which have already been accumulated 

 to go further, and to say that it is probable that the nerves in question 

 were primarily and are still largely developed independently of the 

 central nervous system from a series of intersegmental ectodermal 

 ganglia and that the connexion with the brain and spinal cord is a 

 secondary one. The ganglia became connected together by longitudinal 

 commissures forming a chain of ganglia on each side, and extending 

 to a common meeting place in front of the brain at all events in 

 the Cyclostomes. Those in the body lost their connexion with the 

 spinal cord, but retained their relationship with the ectoderm, thus 

 forming the nerve and the organs of the lateral line. Anteriorly the 

 nerve of the lateral line is continuous with the vagus, and the latter, 

 as has been said, was originally and still is to some extent connected 

 with the nerves in front. In the head region the ganglia and the 

 nerves arising from them attained a conspicuous development, estab- 

 lishing the organs of sense, the sensory, and with few exceptions the 

 motor nervous system of the region, and extending in certain cases 

 beyond it. 



The growth of the ectodermal system of nerves led to the gradual 

 suppression in an antero-posterior direction of the later arising dorsal 

 roots of an altogether or almost entirely sensory type, even as has been 

 stated affecting the first nerve of the spinal series. But these per- 

 sisted in the body region, where the nerves of the mixed type had 

 already taken on another function. The ventral roots and the myo- 

 tomes have suffered a similar but not so marked reduction in the 

 cranial region. 



I see reason for concluding that an early transitory attempt at 

 a lateral line formation takes place in the gull, in other birds, reptiles 

 and mammals. The lateral (paraneural) crest of the trunk which is 

 clearly marked in the gull, but which I have not figured here (the 

 feature is so well known that the omission is warranted), presents 

 intersegmental swellings and may be followed up to the vagus region 

 at a stage when the ganglia of the latter are appearing. The lateral 



