STRUCTURAL MODIFICATIONS 141 



organization has attained the greatest development, namely 

 the Vertebrates, to which man himself belongs. The body 

 remains symmetrical. The structure of the vertebral column, 

 even the arrangement of the muscles in the body-walls — 

 especially among Fishes and Batrachians — of the nerves, 

 blood-vessels, and lateral sense-organs in the aquatic Verte- 

 brates, of the renal ducts in those Vertebrates provided with 

 gills at birth, and of the embryos of those where these organs 

 are vestigial and disappear before birth — all this leaves no 

 doubt about the relationship of the Vertebrates to segmented 

 animals, to which Etienne Geoffroy Saint-Hilaire, 1 Semper, 2 

 and Balfour 3 drew attention. Even in 1869, however, 

 de Lacaze Duthiers insisted that there was an unbridgeable 

 gulf between the Invertebrates and the Vertebrates. It is, 

 of course, true that the Vertebrates entirely lack the 

 characteristic arrangement of the nervous system found in all 

 Invertebrates in which a nervous system is differentiated, 

 — the ganglionic ring surrounding the beginning of the 

 oesophagus ; furthermore, while the nerve-chain which, in 

 Invertebrates, is usually a continuation of this ring, is ventral, 

 the spinal cord, which seems to correspond to it among 

 the Vertebrates, is dorsal. Conversely, the circulatory 

 centre is dorsal in the segmented Invertebrates, and ventral 

 in the Vertebrates. Etienne Geoffroy Saint-Hilaire had already 

 pointed out, in 1808, that this opposition was only apparent 

 and that in order to make it disappear it was only necessary 

 to place the segmented Invertebrates back downwards and 

 belly upwards — that is, to reverse their attitude. But why 

 this reversal ? Geoffroy Saint-Hilaire limited himself to 

 envisaging the matter from the point of view of unity of plan 

 in the composition of the Animal Kingdom, and though the 

 idea seemed to be ingenious and even drew an anonymous 

 letter from the physicist Ampere raising certain difficulties, 

 it was soon abandoned. Yet the realitv of the inverted attitude 

 suggested by Geoffroy can be both demonstrated and 

 explained. In origin, the oesophageal ring of the segmented 

 animals was simply the result of a sensitive differentiation of 

 the epithelial cells surrounding the mouth, brought about 

 by the stimulating action of the food seized and swallowed by 

 the organism. We can follow, as we have seen (p. 122) in the 



1 L. - LI. 3 LII. 



