Johnston, Morphology of the Head. 275 



PLATE II. 



Fig. 8, A, B, C. — Three diagrams to illustrate the hypothesis of the origin 

 of the acustico-lateral system from the margin of the neural plate. Each func- 

 tional division of the nervous system is represented by one cell and fiber. Several 

 general cutaneous ganglion cells which are destined to form the acustico-lateral 

 ganglion are shown. The mesoderm and somatic motor nerve are omitted from 

 C, because by the time the acustico-lateral system has reached the grade of 

 development represented, somite 4 no longer forms a myotome. 



PLATE III. 



Fig. g, A-G. — .Seven outline diagrams to illustrate the hypothesis of the 

 origin of the eye by modification of a general cutaneous ganglion and its corres- 

 ponding centers. The retinal area is indicated by a row of large dots. The 

 relation of the inner wall of the optic vesicle to the choroid plexus, spoken of in 

 the text, is shown in F and G. A comparison of Fig. 9 F with Fig. 4 will 

 show the similarity of position between the retina and acusticum. It will also 

 show that the optic tract fiber from the retina takes a course corresponding to 

 that of the secondary fiber (internal arcuate) from the cutaneous nucleus. 



PLATE IV. 



Fig. 10. — A diagram of segmentation in a generalized vertebrate head. In 

 its general features the diagram follows Petromyzon more closely than any other 

 form. So, the number of the gill clefts, their position relative to the somites, the 

 position of the auditory vesicle, and the formation of hypobranchial muscles from 

 myotome 10 and following, are taken from Petromyzon. The neuromeres, the 

 nerve of LocY, the nervus thalamicus, the relation of the head lines to the audi- 

 tory 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. 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 continued caudally 

 until it came into connection with the viscero-motor nucleus of the trunk nerve. 

 The accessorius nerve is not shown. The somatic nucleus and root are shown 

 for all the somites except somite 4, where they are shown in dotted outline. The 

 nerves for both somites 4 and 5 are absent in Petromyzon. Probably both are 

 present in Bdellostoma (a'v and /5^v of Furbringer). It is possible that one or 

 both of these has joined with N. VI in gnathostomes. 



