Origin of the Vertebrates. 253 



reference to the outer world, the heart and the longitudinal 

 nervous axis occupy contrary positions in the Vertebrates and 

 the segmented Invertebrates, so that if we apply the term 

 ventral to the side of the body turned towards the ground, 

 and call the opposite side dorsal, the nervous axis is dorsal in 

 the Vertebrates and ventral in the segmented animals, and 

 the contractile vessels are situated on the side of the body 

 opposite to the nervous system. 



These are the only characters common to all Vertebrates, 

 including Amphioxus; and since no one disputes that all these 

 animals can easily be derived from the simplest forms among 

 them, once these eight groups of characters are explained it 

 must be admitted that the theory of the Vertebrate is com- 

 plete ; conversely, every system of genealogy which should 

 fail to explain these eight groups of characters must be 

 rejected. 



(1) Metamerism of the Vertebrate Body. — Whatever be the 

 cause of metamerism — a cause that we shall examine in a 

 subsequent communication — the formation of the segments in 

 all the metamerically segmented animals is a precocious 

 developmental phenomenon, the mechanism even by which 

 the body is constituted ; by this process many organisms 

 are formed the segments of which may afterwards become 

 obliterated, but an organism not funned by this process once 

 constituted never divides again into well-defined segments. 

 The law of patrogony (repetition of the genealogy by the 

 embryogeny) — a fundamental law universally accepted — is 

 therefore opposed to the possibility of attributing to the Verte- 

 brates an ancestor whose body was not clearly segmented in 

 the adult state, or at the very least multisegmental during 

 the embryonic period. This at once excludes the Nemertiues, 

 Balanoglossus, and the Appendicularida?, and leaves only the 

 Arthropods and the Annelid Worms. In truth, in the 

 embryos of these animals the septa between the segments are 

 complete, while in those of the Vertebrates they are confined 

 to the dorsal half of the body ; but the embryogeny of Amphi- 

 oxus shows that this is a result of tachygenesis or embryogenic 

 acceleration. The segmentation of the embryos of Amphioxus 

 is at first eomplete (Hatschek), and consequently identical 

 with that of the Annelid Worms ; afterwards the ventral 

 portion of the septa is absorbed again ; this condition is 

 realized at once in the Vertebrates proper. 



(2) The Vibratile Cilia. — The entire organization of 

 Arthropods is to some extent dominated by the property 

 which their epithelial elements possess of forming, in their 

 superficial region, a deposit of chitin, which causes this region 



