ox THE ANCKHTRAL FOILM OF THK d I/O U DAT A. 263 



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

 median dorsal line\ 



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 late- 

 ral, and most probably at one time in- 

 dependent, cords ; and as already indi- 

 cated this is the view I am myself in- 

 clined to adopt; vide pp. 251 and 2-52. 



The orig-in and nature of the 

 mouth. The most obvious point con- 

 nected 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, th« Myx- 

 inoids, and some Ganoids, but is lost in 

 other vertebrate forms. A mouth, situ- 

 ated as is the embryonic vertebrate 

 mouth, is very ill adapted for biting ; and 

 though it acquires in this position a 

 distinctly biting character in the Elas- 

 mobranchii, 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 Elasmobranchii. 



On the structure of the primitive mouth there appears to me to 

 be some interesting embryological evidence, to which attention has 

 already been called in the preceding chapters. In a large number 

 of the larvaB or embryos of the lower Vertebrates the mouth has a 

 more or less distinctly suctorial character, and is connected with 

 suctorial organs which may be placed either in front of or behind it. 

 The more important instances of this kind are (1) the Tadpoles of 

 the Anura, with their posteriorly placed suctorial disc, (2) Lepidosteus 

 iarva (fig. 195) with its anteriorly placed suctorial disc, (3) the 

 adhesive papillae of the larvae of the Tunicata. To these may be 

 added the suctorial mouth of the Mvxinoid fishes^ 



Fig. 194. Tkansvehse sec- 

 tion THROUGH THE TRUNK OF AN 

 EMBRYO SLIG3TIiY OLDER TH>N 

 FIG. 28 E. 



J!C, neural canal; pr. posterior 

 root of spinal nerve; x. subnoto- 

 chordal rod; ao. aorta; sc. so- 

 matic mesoblast; sp. splanclmic! 

 mesoblast ; mp. muscle-plate ; 

 mp'. portion of muscle-jjlate con- 

 verted into muscle; Vv. portion 

 of the vertebral plate which will 

 give rise to the vertebral bodies; 

 al. alimentary tract. 



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



"^ The existing Myxinoid Fishes aie no doubt degenerate types, as was first clearly 

 pointed out by Dohrn; but at the same time (although Dohrn does not share this view) 

 it appears to me almost certain that they are the remnants of a large and very 

 primitive group, which have very likely been preserved owing to their parasitic or 



