tjik VEi;iT,r,i;.\TA. 



59 



to possess a nervous system and ;i heart, that section mav be 

 represented diagrammatically as in Fig. 30 (I.), where 1* repre- 



l-V. 30. 



.Ar- 



il, m 



jyt- 



Fig. 30. — Diagrams representing generalised sections of* one of the higher I a vertebrates 

 (I. II.), and of a Vertebrate (III. IV.); I. III. transverse, II. IV. longitudinal sec- 

 tion. A, alimentary canal; H, heart; P, parietes of the body; P', parietes of the 

 neural canal; N, nervous centres of Invertebrate; N 1 , sympathetic, and N 2 , cerebro- 

 spinal centres of Vertebrate ; ch, notochord ; M, mouth. 



sents the parietes or wall of the body, A the alimentary canal, H 

 the heart, and N the nervous centres. It will be observed that the 

 alimentary canal is in the middle, the principal centres of the 

 nervous system upon one side of it, and the heart upon the 

 other. In none of these animals, again, would you discover, in 

 the embryonic state, any partition, formed by the original ex- 

 ternal parietes of the body, between the nervous centres and the 

 alimentary canal. 



But, in the five vertebrate classes, the parietal portion of the 

 blastoderm of the embryo always becomes raised up, upon each 

 side of the middle line, into a ridge, so that a long groove is 

 formed between the parallel ridges thus developed ; and the 

 margins of these, eventually uniting with one another, constitute 

 a second tube parallel with the first, by a modification of the 

 inner walls of which the vertebrate cerebro-spinal nervous 

 centres are developed. Hence it follows that, after any verte- 

 brated animal has passed through the very earliest stages of its 

 development, it is not a single, but a double tube, and the two 



