STO.MODAEUM AND MEDULLARY TUBE 1 1 



will see that in ontogeny this backward shifting of the 

 blastopore, being the future neurenteric canal, is quite evident. 

 We then have to assume that the stomodaeum has lost its 

 original function and a new, secondary mouth had to be 

 formed. This gives us at the same time the solution of 

 the problem of "the old mouth and the new" (BEARD, 

 1888. a), cf the palaeostoma and the neostoma of KUPFFER 

 (1894) and shows us clearly, how the old mouth could 

 have been lost and why a new one had to be formed in 

 a way which reminds us of the Deuterostomia. This new 

 mouth accordingly breaks through only very late in embryonic 

 life, as DOHRN first emphasized (see quotation above). 

 Moreover we will see in the second chapter that the 

 new mouth in Ascidians, Amphwxus and Craniates is formed 

 in three different ways and, as a consequence, is not homo- 

 logous in these three groups. This points equally to the 

 secondary nature of the Vertebrate mouth. 



Nevropore. — The primary mouth, being that of the Anne- 

 lias, is represented bv the neuropore of Amphioxus, and 

 this itself again is phylogenetically secondary in respect 

 to the „Urmund", the mouth of the hydroid polyps, which 

 in Annelids we find again in the cardiac pore, and in 

 Chordates in the neurenteric canal, both representing the 

 former blastopore. Thus in the ontogeny of Vertebrates 

 we see the three successive mouths appear in the same 

 succession as they appeared in phylogeny : the blastopore 

 ("Urmund"), Ihe neuropore (the Annelidan mouth) and 

 finally the definitive mouth. 



Gaskell. — It truly seems a bold supposition that a part 

 of the alimentary canal should have changed its function 

 and have become the central nervous system. Yet, as we 

 will see especially in the last chapter, the facts of embryo- 

 logy plead so strongly for it that KOWALEWSKY and the 

 writer are not alone in their idea. A somewhat similar 

 suggestion has been made by the physiologist GASKELL (1908) 

 in his theory on the origin of Vertebrates. More than once, 

 in conversation and in correspondence, 1 have heard my theory 

 compared with that of GASKELL, a comparison which 1 must 

 add at once did not exactlji flatter me. I readily believe 

 that Gaskell was a good physiologist, but I cannot 

 admire his phyiogenetic speculations, to which I think my 

 theory bears only a superficial resemblance. GASKELL derives 

 the Vertebrates from Arthropod-like ancestors by making . 



