THE VERTEBRATA. 61 



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 you, 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. I have taken some 

 pains to verify the remarkable discoveries of Eemak in relation 

 to this point, and, so far as the chick is concerned, his state- 

 ments 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 Vertebrata and the ganglionated nervous 

 centres of the Invertebrata, and the homologues of the latter 

 must probably be sought in the sympathetic. 



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 

 of the nervous system, and some of the ganglia of the latter lie 

 in front of, and others behind, the oesophagus. A longitudinal 

 vertical section of such an animal, therefore, may be represented 

 by Fig. 32 (II.). 



A similar section of a vertebrated animal shows, on the con- 

 trary, the chief centre of the nervous system not to be perforated 

 by the oesophagus ; the latter turning away from it and opening 

 upon the opposite side of the body (Fig. 32, IV.). 



Another structure sharply distinctive of the vertebrate 

 classes is the chorda dorsalis or notochord, an organ of which 

 no trace has yet been discovered in any of the inverte- 

 brates, though it invariably exists, in early embryonic life at 

 least, in every vertebrate. Before the cerebro-spinal canal is 



