576 CEPHALOCHORDA. 



similarity of structure between the primitive kidney-tubules of the 

 Selachian embryo and the segmental organs of the Annelida. Our 

 acceptance of this homology, however, has been recently made im- 

 possible by the researches of van Wijhe (No. 48), Kuckert 

 (No. 44), and Boveri (No. 2) which cause us to doubt the serial 

 homology of the pronephros with the primitive kidney and to regard 

 the latter as a secondary acquisition which in any case has nothing 

 to do with the segmental organs. From this point of view it would 

 be the pronephros which requires consideration. 



The derivation of the Vertebrates from the Annelida is, in fact, 

 accompanied by certain difficulties which are not insurmountable. 

 The most important of these is the position of the mouth. While, 

 in the Annelida, the oesophagus perforates the central nervous system 

 at the oesphageal ring in such a way that the supra-oesophageal 

 ganglion comes to lie above and the ventral cord below the gut, 

 no such relation between the stomodaeum and the central nervous 

 system is to be found in the Vertebrates. The most varied hypotheses 

 have been suggested to obviate this difficulty. Schneider thought 

 that he could discover in the connection of the hypoglossal and tri- 

 geminal nerves an equivalent for the oesophageal ring, van Beneden 

 and Julin (No. 29) have assumed that, in the primitive ancestors 

 of the Vertebrates, the oesophagus shifted forward in the median 

 plane between the still unconnected cephalic lobes of the brain, while 

 other zoologists like Kleinenberg, Beard, No. 27, v. Kennel 

 (No. 35a) believe that the supra-oesophageal ganglion atrophied and 

 that the brain and the dorsal chord of the Vertebrates only are the 

 equivalents of the ventral cord of the Annelida. In connection 

 with this idea, we have the question as to the primitive mouth of 

 the Vertebrates. Under the assumption that the pro-cephalon of 

 the Vertebrates corresponds to the supra-oesophageal ganglion of these 

 Annelida, we should have to conjecture that the original vertebrate 

 mouth, which perforated the oesophageal ring, lias disappeared. The 

 definitive mouth of the Vertebrates would have to be regarded as a 

 secondary formation, and is by many zoologists thought to have arisen 

 through the fusion of gill-slits. There is actually much to support 

 the view that the mandibular arch is a branchial arch which has 

 been drawn into the mouth. This, however, does not decide the 

 question of the derivation of the vertebrate mouth from gill-clefts. 

 In the case of Amphioxus, there is no ontogenetic indication of the 

 derivation of the mouth from gill-clefts. The mouth here arises in a 

 different manner from the gill-slits (p. 550) and in a different position. 



