GO ON CLASSIFICATION. 



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 clia grammatically by Fig. 30 

 (III.), w T here, for the most part, the letters have the same sig-nifi- 

 cation as in the fore^'oiiis; case, but where P' denotes the 

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

 as in the case of the invertebrate animal, the alimentarv canal, 

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

 sympathetic system. This nervous system and the heart are 

 situated upon opposite sides of the alimentary canal, the sympa- 

 thetic corresponding in position, and in forming a double chain 

 of ganglia, with the chief nervous centres of the invertebrate ; so 

 that the cerebro-spinal tube appears to be a super-addition, — a 

 something not represented in the invertebrate series. The 

 formation of the cerebro-spinal tube of vertebrates, in the 

 manner which I have described to von, is a well-established 

 fact ; nor do I entertain any doubt that the cerebro-spinal 

 centres, viz., the brain and the spinal cord of vertebrates, are the 

 result of a modification of that serous layer of the germ which 

 is continuous elsewhere with the epidermis. Two years ago, 

 I took some pains to verify the remarkable discoveries of 

 Remak in relation to this point, and, so far as the chick is 

 concerned, his statements appeared to me to be fully borne 

 out. But, as Von Baer long ago suggested, it is a necessary 

 result of these facts that there can be no comparison between 

 the cerebro-spinal nervous centres of the Verttbrata and the 

 ganglionated nervous centres of the Invertebrata, and the 

 homologues of the latter must probably be suught in the sym- 

 pathetic. 



Doubtless in close connection with this profound difference 

 between the chief nervous centres of the vertebrate and the 

 invertebrate is another remarkable structural contrast. In all 

 the higher invertebrates, with a well-developed nervous system, 

 the latter is perforated by the gullet, so that the mouth is 

 situated upon the same side of the body as the principal masses 

 • •I' the nervous system, and some of the ganglia of the latter lie 



