216 THE ANCESTRY OF VERTEBRATES 



been transformed into an elastic supporting organ, the 

 notochord. In the tentacles also of Cnidarians we see the 

 endoderm produce large vesicular cells constituting a sup- 

 porting tissue which here, truly, has not the elasticity of 

 the notochord. Thus 1 think the notochord is a derivative 

 from the endoderm in the same way as the mesoderm but 

 of phylogenetically younger origin than the latter. In onto- 

 geny also do we see that the development of the mesoderm 

 bands in Chordates begins and is completed earlier than 

 is that of the notochord. From this circumstance GOETTE 

 (1890, p. 26) concludes: "dass die Chordabildungerworben 

 wurde, nachdem die bilaterale Mesodermbildung bei den 

 Vorfahren der Chordaten bereits bestand." It is difficult 

 to say whether we should consider the notochord as meso- 

 derm or endoderm. 



Gill-slits. — As regards the gill-slits, we look once more 

 in vain for an homologous structure in Annelids. In the 

 chapter on the head, however, I have compared them with 

 the paired diverticula of the gut in flatworms and nemer- 

 tines. For this I refer to p. 82. It will be evident that the 

 gill-slits of Balanoglossus, formed from the ectodermal stomo- 

 daeum, can have no connection with those of Vertebrates 

 and can only be considered as analogous structures. The 

 Deuterostomia can not give us the key to the solution of the 

 problem of the origin of Vertebrates and to the explanation 

 of the structure of their organs. 



