236 



COMPARATIVE ANATOMY 



(placodcs) take part, in addition to the main ganglia already referred 

 to. Two rows of these accessory ganglia (which probably corre- 

 spond to the vestiges of primitive in- 

 tegumentary sense-organs) may often 

 be distinguished in the embryo in 

 either side of the head a dorso-lateral, 

 and a more ventral row just above the 

 gill-clefts : the former are spoken of as 

 the lateral ganglia, and the latter as 

 the ventral or epibranchial ganglia, 

 which are associated with transitory 

 epibranchial sense-organs. In their 

 primitive superficial position the two 

 rows are connected with one another and 

 with the central organ by cellular cords. 

 It is probable that the motor 

 fibres of these mixed nerves (as well 

 as the cerebral portion of XI) grow 

 out secondarily from the brain into 

 the primary ganglionic nerve-rudi- 

 ments arising from the upper row of 

 nerve-centres formed by the bifurca- 

 tion of the band-like spinal motor 

 tract. From the ventral row, which 

 lies in the same plane as the ventral 

 horns of gray matter, arise the Illrd, 

 IVth, Vlth, and Xllth nerves, as 

 already mentioned. 1 



It must be remembered that the 

 head is primitively composed of a 

 series of metameres (p. 75), and it 

 is therefore important to ascertain, as 

 far as is possible in the present state 

 of our knowledge, to which individual 

 metameres the different cerebral nerves 

 belong. As already mentioned, the 

 olfactory and optic nerves present cer- 

 tain peculiarities which bring them 

 under another category, and they will 

 be treated of later in connection with 

 the corresponding sensory organs. 

 The following general summary gives a scheme of the prob- 

 able primitive relations of the head-segments and cerebral nerves, 



1 An additional pair of vestigial cerebral nerves has been shown to exist in 

 Protopterus, Ceratodus, Anura, and many Elasmobranchs, arising in the region of 

 the lamina terminalis, crossing the olfactory lobes, and ending in the region of 

 the olfactory mucous membrane. Along the course of this nerve, the function 

 of which is unknown and which has been described as the " nervus terminalis," 

 are one or two ganglia. 



FIG. 178. DIAGRAM TO SHOW 

 THE MODE OF DEVELOPMENT 

 OF THE DORSAL CEREBRAL 

 NERVES AND THEIR GANGLIA 

 IN THE LAMPREY. (After 

 C. v. Kupffer.) 



ch, notochord ; d, gut ; ge, epi- 

 branchial ganglion (ventral 

 placode) ; gl, lateral vagus 

 ganglion (lateral placode) ; 

 gs, sympathetic ganglion ; 

 h, hind-brain ; I, neural ridge 

 (rudimentof the spinal gang- 

 lion proper) ; m, mesoderm ; 

 nb, branchial nerve : nd, 

 subepidermoid layer,derived 

 from the ectoderm, and giv- 

 ing rise to the peripheral 

 part of the branchial nerve ; 

 us, dorsal spinal nerve. 



