STOMODAEUiM AND MEDULLARY TUBE 21 



and the soft palate, and are innervated by the gustatoyr 

 branches of the N. trigeminus, facialis and glossopharyngeus. 



Protostomia, Deuterostomia, Tritostomia. — We have come 

 to the conclusion that the old distinction by Hatschek of 

 the three groups Zygoneura, Ambulacralia and Chordonia 

 is to be piefered to Grobben's division into Proto- and 

 Deuterostomia, since the latter group does not constitute a 

 natural unit. HATSCHEK and his assistant SCHNEIDER have 

 also been inclined to unite their Ambulacralia and Chor- 

 donia into one group, the Enterocoela, in which the 

 coelomic mesoderm is of entodermal origin, whereas in the 

 Zygoneura or Ecterocoela it would be of ectodermal origin 

 (cf. HATSCHEK, ISll, p. 18). That this last opinion is erro- 

 neous has been conclusively demonstrated by the cell-lineage 

 investigations. 



My theory, on the contrary, would rather point into the 

 direction of a union of the Chordonia with the Zygoneura, 

 from which they are to be derived. Yet a subdivision of 

 the "Coelomata", the three-layered Metazoa, into three groups 

 is no doubt the more preferable method. In GROBBEN's 

 nomenclature we could designate these three groups as 

 Protostomia, Deuterostomia and Tritostomia. 

 In the first group the primordial opening of the gut to the 

 exterior, the blastopore, becomes the ingestion-opening. In 

 the second group it passes into the anus and a second 

 opening becomes the mouth. In the Tritostomia the first 

 and the second opening pass into the canalis neurentericus 

 (former ingestion-opening) and the anus, while the mouth 

 breaks through only late in embryonic life as a third opening. 

 To the question if there is any relation of the anus to the 

 blastopore we will revert in the last chapter. There I hope 

 to show that not only the application of the principles of 

 my theory will bring us the solution of the old problem of 

 the relation between anus and blastopore, which will prove 

 to be a secondary one, but also that, considered in the light 

 of this solution, the facts to be observed yield one of the 

 strongest supports in favour of the view that the neural 

 tube indeed represents nothing but the former stomodaeum 

 that has grown out in caudal direction as far as the pos- 

 terior end of the body and even further still (formation of 

 the tail). 



