ON THE ANCESTRAL FORM OF THE CHORDATA. 



317 



the posterior roots arising from the extreme dorsal line (fig. 194), 

 a position which can most easily be explained on the supposition 

 that the two sides of the plate, from which the nerves originally 

 proceeded have been folded up so as to meet each other in the 

 median dorsal line 1 . 



The medullary plate, before becoming folded to form the 

 medullary groove, is (except in Amphibia) without any indication 

 of being composed of two halves. In both the embryo and 

 adult the walls of the tube have however a structure which points 

 to their having arisen from the coalescence of two lateral, and 

 most probably at one time inde- 

 pendent, cords ; and as already indi- 

 cated this is the view I am myself in- 

 clined to adopt ; vide pp. 303 and 



304- 



The origin and nature of the 



mouth. The most obvious point 

 connected with the development of 

 the mouth is the fact that in all 

 vertebrate embryos it is placed 

 ventrally, at some little distance 

 from the front end of the body. 

 This feature is retained in the adult 

 stage in Elasmobranchii, the Myx- 

 inoids, and some Ganoids, but is lost 

 in other vertebrate forms. A mouth, 

 situated as is the embryonic verte- 

 brate mouth, is very ill adapted for 

 biting ; and though it acquires in 

 this position a distinctly biting cha- 

 racter in the Elasmobranchii, yet it 

 is almost certain that it had not such 

 a character in the ancestral Chordata, 

 and that its terminal position in 

 higher types indicates a step in advance of the Elasmo- 

 branchii. 



On the structure of the primitive mouth there appears to me 



FIG. 194. TRANSVERSE SEC- 

 TION THROUGH THE TRUNK OF 

 AN EMBRYO SLIGHTLY OLDER 

 THAN FIG. 28 E. 



nc. neural canal ; pr. posterior 

 root of spinal nerve ; x. subnoto- 

 chordal rod ; ao. aorta ; sc. so- 

 matic mesoblast ; sp. splanchnic 

 mesoblast ; mp. muscle-plate ; 

 tup', portion of muscle-plate con- 

 verted into muscle ; Vv. portion 

 of the vertebral plate which will 

 give rise to the vertebral bodies ; 

 al. alimentary tract. 



1 Vide for further details the chapter on the nervous system. 



