Origin of the Vertebrates. 257 



eliminated area, the origin of which is quite clear, which, by 

 the grouping of its elements into a cord, constitutes the noto- 

 chord The cord thus constituted has nothing except the 

 mortification of its cells in common with the endodermic 

 diverticulum which penetrates into the proboscis of Bulaao- 

 glossus and which has its analogue in the case of Cephalo- 

 discu*. These structures can be no more likened to a noto- 

 chordal rudiment than could the median diverticulum, entirely 

 analogous this time to that in Balanoglossus, which is thrown 

 out in front by the main intestine of the majority of the poly- 

 clad and triclad Turbellarians. On the contrary, it will 

 easily be understood how it has been possible for a notochord 

 to be formed in an Annelid Worm, of which the nervous 

 system, developed to an exceptional degree, and the meso- 

 dermic rudiments have been affected by tachygenesis. Now 

 the formation of the nervous system by an invagination of 

 the ectoderm and that of the mesoderm by two evaginations 

 of the endoderm are evident proofs of the intervention of 

 tachygenesis *. The considerable proportions assumed by 

 the nervous system furnish, moreover, the explanation of the 

 two latter distinctive characteristics of the Vertebrates. 



(7) Disappearance of the (Esophageal Ring. — The excep- 

 tional volume assumed by the nervous system in all Verte- 

 brates excludes from their genealogy Balanoglossus, in which, 

 on the contrary, the axial cord is reduced to a very short 

 rudiment which does not exceed the collar in length. The 

 proportions to which the nervous axis attains necessarily 

 induce its precocious development, its formation by rapid 

 processes. It originates, in fact, at the expense of a fairly 

 large area of the ectoderm, which buries itself beneath the 

 neighbouring regions at a period well in advance of that at 

 which the mouth is constituted. The nervous system being 

 outlined before the mouth has no longer to concern itself with 

 the existence of the latter, as it does in the case of the 

 Annelid Worms, by developing around it. There is no 

 longer a raison d'etre for the oesophageal ring f ; it disap- 

 pears, and the brain proceeding to its completion on the 

 median neural line stands in the way of the formation of the 

 mouth upon this line. It is the cause of the reversal of the 

 attitude of the Vertebrates, as already pointed out by 

 Geoffroy tSaint-Hilaire. 



($) Reversal of Attitude in the Vertebrate. — The mere com- 

 parison of a section of the embryo of a dog-fish and of a 



* See ' Comptes Rendus,' December 1896 : Report on the Competition 

 for the Serres Prize, by the author. 

 t E. Perrier, ' Les Colonies auimales,' p. 695 (1881), 



