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Neuropore of Craniates and of Amphioxus. — Thus only in 
Amphioxus the neuropore corresponds to the mouth of the 
Annelids and represents the old mouth of the Vertebrates. 
In the development of Craniates we shall see the old mouth 
appear only in cases where the closure of the medullary 
tube precedes that of the cerebral plate so that the prae- 
chordal cerebral plate, with the optic pits, is still open 
when the hindbrain and the spinal cord have already 
closed to a tube which opens to the exterior at its 
fore-end with a kind of provisional neuropore, as KEIBEL 
figures for the pig (fig. 8). This provisory neuropore, at 
the place where later the isthmus will be found, here 
represents the old mouth. Perhaps also the thin roof of 
the fossa rhomboidea is to be regarded as a trace of the 
former mouth. 
Cranial flexure. — At the same time the almost invariable 
appearance of the so-called cranial flexure or “Kopfbeuge” 
between the praechordal and the epichordal part of the 
brain may be explained as corresponding to the angle 
between apical plate and stomodaeum in Annelidan larvae. 
In Amphioxus, as might be expected, we do not find a 
trace of such a flexure. 
Auditory organs in Acrania. — The conclusion, reached 
along three different paths, of the homology of fore-brain 
and part of the apical plate, will be confirmed by further 
considerations. Having traced back the eyes of Vertebrates 
to those of the Protostomia, we shall now turn our attention 
to the auditory and olfactory organs. The auditory organs of 
Craniates can in no way be derived from those of Tunicates. 
In Ascidians we find, close to the eye, a statolith on a little stalk 
in the interior of the brain vesicle. The statocyst of Appen- 
dicularians originates as well from the brain vesicle which 
by one-sided thickening of the wall gives rise to the cerebral 
ganglion, at the left side of which the statocyst, evidently 
comparable to the sense-vesicle of Ascidian larvae, is situated 
(DELSMAN, 1912). In both cases the ie of equilibrium is 
of encephalogenetic origin. Amphioxus has no such organ. 
Organs of equilibrium of Annelids ip Molluscs. — With the 
organs of equilibrium of worms and molluscs the internal ear 
of Craniates exhibits an undeniable affinity. In both cases 
we have a paired structure originating as two vesicles which 
invaginate from the epidermis. Sometimes there remains a 
communication with the exterior in the form of longer or 
