ORIGIN AND STRUCTURE OF THE HEAD 39 



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 organ of equilibrium is 

 of encephalogenetic origin. Amphioxus has no such organ. 



Organs of equilibrium of Annelids and 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 



