60 



INTRODUCTION TO CLASSIFICATION. 



blastoderm of the embryo always becomes raised up, upon each 

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



Fig. 32. Diagrams representing generalised sections of one of tlie higher Invertebrates 

 (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 boily ; P', parietes of the 

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

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



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 

 tubes are separated by a partition which was, primitively, a part 

 of the external parietes of the body, but which now lies, in a 

 central position, between the cerebro-spinal nervous centres and 

 the alimentary canal. Hence, a transverse section of any verte- 

 brated animal may be represented diagrammatically by Fig. 32 

 (III.), where, for the most part, the letters have the same signifi- 

 cation as in the foregoing case, but where F denotes the second, 

 or cerebro-spinal, tube. The visceral tube (P) contains, as in 

 the case of the invertebrate animal, the alimentary canal, the 

 heart, and certain nervous centres, belonging to the so-called 

 sympathetic system. This nervous system and the heart are 



