72 NERVOUS SYSTEM OF VERTEBRATES. 



terminalis, the nervus thalamicus, the relation of the lateral lines of the head to the 

 auditory vesicle, and the praeoral entoderm are taken from selachians. The sensory 

 nerve roots are represented as retaining their attachment to the dorsal surface of 

 the neural tube where they were formed from the neural crest. The segmental 

 position of these roots is about that which they have in the embryo of Petromyzon, 

 except the root of N. X, which has been shifted back a little farther than it is in 

 Petromyzon. The general cutaneous nerve shown in dotted outline over somite 6 

 is the Vagusanhang of HATSCHEK in Ammocoetes and the nerve which unites 

 with the vagus root in the embryo of selachians. This is the second dorsal spinal 

 nerve in Petromyzon dorsatus and other primitive cyclostomes. The position of 

 the viscero-motor nuclei somewhat caudal to the several roots is indicated. The 

 viscero-motor nucleus of the vagus and accessorius is shown as a single' large 

 nucleus extending through two segments. It might more properly have been con- 

 tinued caudally until it came into connection with the viscero-motor nucleus of the 

 trunk nerve. The accessorius nerve is not shown. The somatic motor nucleus 

 and root are shown for all the somites except somite 4, where they are shown in 

 dotted outline. The nerve for somite 4 is absent in Petromyzon, but is present in 

 Bdellostoma. It is possible that this root has joined with N. VI in gnathostomes. 



The process of cephalization in vertebrates has consisted chiefly 

 in (i) the development of special sense organs; (2) the consequent 

 enlargement of the brain; (3) the formation of a rigid cranium 

 to protect sense organs and brain; (4) the disappearance of certain 

 muscle segments and the change of position and function of others 

 (eye- muscles) ; (5) the reduction in number of gills, the formation 

 of a new mouth and the expansion of the persistence gills; (6) the 

 disappearance of various nerves owing to the reduction of somites 

 and gills; (7) the shifting of position of various organs and nerve 

 roots due to these changes; and (8) the great development in 

 higher vertebrates of the so-called higher brain centers. At every 

 step in the evolution of vertebrates these changes have left their 

 impression on the nervous system so that the nervous system to 

 a greater extent than any or all other organs preserves a record 

 of the course of phylogenetic development. For this reason not 

 only does a knowledge of the nervous system throw light upon 

 the morphology of the head, but a knowledge of the evolution of 

 the head is necessary for a true understanding of the nervous 

 system. 



TABLE A. 



Shifting of organs during development of selachian embryo. 



Length of embryo 12^-13 mm. 40 mm. 



Duct of Cuvier shifts from trunk segment 2 to trunk segment 10 



Omphalo-mesenteric artery " .... 4- 5 " 12-13 



Sixth branchial artery, from occipital segment . 3 o 



Ostium of Muller's duct, from trunk segment .. 5-6 " n 



