58 THE ANCESTRY OF VERTEBRATES 



Annelids and Vertebrates, came to an opposite opinion^ 

 after it had been demonstrated that in Vertebrates several 

 of the cranial ganglia originate partly from the neural ridge 

 and partly from the ectoderm, viz : from gang,lionic prolifer- 

 ations of either transient or permanent lateral organs. He 

 now assumed that originally the lateral organs had some 

 physiological connection with the gills, over which they are 

 situated in the cranial region and which are innervated from 

 their ganglia. He therefore changes their name into branchial 

 sense-organs. Ontogenetically we first see the lateral organs 

 appear on the head and then, together with the outgrowing 

 ramus lateralis vas[i, extend backwards over the trunk. From 

 this mode of development BEARD deems it probable, that they 

 were originally restricted to the gill bearing region and from 

 here have only secondaiily spread over the trunk. "Damit 

 wird aber", EISIG infers, "eine ontogenetische Thatsache 

 willkUrlich, das heisst ohne Beriicksichtigung aller im Wege 

 stehenden Schwierigkeiten. ins Phylogenetische ubersetzt. 

 Und die Hauptschwierigkeit bestehi darin, plausibel machen 

 zu konnen. wie denn eigentlich dieses ursprunglich allein 

 am Kopfe entwickelte Seitenorgansystem dazu kommen 

 sollte, sich secundar in segmentaler Anordnung auf den 

 Rumpf auszudehnen, auf denjenigen Korpertheil der doch 

 notorisch als der phylogenetisch altere und einfachere zu 

 betrachten ist". This latter opinion we cannot share from 

 the point of view of our theory, the soma of Annelids and 

 Vertebrates is no doubt phylogenetically younger than 

 the pros'omium and not older than the segmented part of 

 the head which, as we shall see, in several respects has 

 even retained a more primitive character than the rest of 

 the trunk. Yet in the spreading of organs, originally 

 restricted to the foremost part of the body, over the trunk 

 and in their assuming a metameric arrangement BEARD also 

 acknowledges a curious fact. The ontogenetic development 

 of the lateral organs from the head backwards over the 

 trunk is accounted for by ElSIG in this way, tnat the former 

 condition, in which they originated independently in all 

 segments, is no longer recapitulated in the trunk in conse- 

 quence of the reduction of their segmental innervation, and 

 that they now arise in connection with the collector-nerve 

 from front to back. 



In any case, the homology of the lateral organs in Verte- 

 brates and Annelids remains somewhat questionable, and it 



