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the whole intestinal tract of the latter, from the mouth to 
the anus, pass into the neural tube of the former. the 
stomach giving rise to the brain and the intestine to the 
spinal canal, while the infundibulum marks the place of 
the old mouth. A new alimentary canal to replace the lost 
„one is formed from a median groove of the ventral body wall 
which by the growing out of two lateral folds is converted 
into a tube. Of course GASKELL is engaged in grave conflict 
with the doctrine of the germinal layers — the central nerv- 
ous system of Vertebrates would be of entodermal *), the 
gut of ectodermal origin! — and with the other princirles 
of embryology. GASKELL truly tries to show, that his theory 
oes not contravene these principles, but he can do so only 
by expressing as his opinion, that “the cmbryologists have 
to a large extent gone wrong in their fundamental princi- 
ples” (p. 459) and by rejecting the doctrine of the homology 
„of the two primary germinal layers in Metazoa. 1 need not 
emphasize, that my theory on the contrary is in perfect 
accordance with the doctrine of the germinal layers; as is 
the general rule in the animal kingdom the nervous system 
in Chordates is of ectodermal origin. Almost the only point 
in which | agree with GASKELL is “that the clue to the 
origin of vertebrates is to be found in the tubular nature 
of the central nervous system of the vertebrate“; however, 
GASKELL and the writer differ widely in their application 
of this principle 
o the question as to what may have been the cause 
„of such a remarkable change of function as is the conver- 
sion of the stomodaeum of the Protostomia into the neural 
tube of the Chordates, the answer is not easily given. Some 
reflections perhaps will assist in making us a little more 
familiar with the idea. 
Conversion of stomodaeum into medullary tube. — In the first 
place the question can be asked: what is the original 
significance and function of the stom>daeum? Why did not 
the blastopore, the “Urmund”, as e. g. in hydroid polyps, 
remain also in higher forms the definitive mouth. Why 
was an ectodermal entrance to the gut formed in connection 
_to it? Partly no doubt this finds its explanation in the 
dense coat of cilia investing the inner surface of the stomo- 
1) In part at least, for according to GASKELL the ganglia of the 
Arthropod ancestors have applied themselves to and enclosed the 
—alimentary canal, forming together with it the neural tube. 
