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medullary suture and ends anteriorly in the neuropore, where- 
it meets the transverse cerebral fold (fig. 5). In the Axolotl, 
however, as suggested above, the anterior end of the cerebral: 
plate and the animal pole are pushed forward by the preco- 
cious longitudinal growth of the medullary plate. They push 
aside to the left and to the right the praecerebral part of 
the apical plate, until they have reached nearly the anterior 
border which, as | will try to demonstrate below (cf. anon), 
probably corresponds to the place of the future mouth. 
By this process the precerebral part of the apical plate has 
been nearly divided into two halves, lying on either side of the- 
cerebral p'ate which indeed reaches here much further in 
front of the anterior end of the notochord than in Anurans 
(cf. the figures). From this circumstance we can now easily 
understand, that the closure of the cerebral plate is performed 
here almost entirely by the growing over of the lateral: 
cerebral folds, that the place of the neuropore is much 
more terminal than in Rana and that the brain gives much 
more the impression of a tube. In this way the different 
results of His and KUPFFER are probably to be explained. 
Kupffer’s view to be preferred to that of His. — But there 
still remains the question as to where the brain axis ends. 
From the point of view of my theory it seems questionable 
indeed, whether we can speak of a brain axis, and whether 
the brain represents a tube which is the continuation of 
the medullary tube. If we start from an Acraniate ancestor 
in which no fore-brain had yet been formed, as in Amphioxus, 
but not so degenerated in certain respects as the latter, 
we might expect to find here in front of the neuropore, 
which corresponds to that of Amphioxus and to the Annelid 
mouth, and on the surface of the prostomium, a pair of 
eye-pits like those of Annelids and Molluscs. These pits 
tended to become deeper and to disappear from the surface, 
as may be observed in the ontogenetic development of the 
eyes of Annelids and Molluscs. Thus it may have happened, 
that they have been involved into a common invagination 
which gave rise to the fore-brain vesicle, but it may be 
doubted if this represents the forward continuation of the 
medullary tube. Another conception is, that the medullary 
tube has extended forward and incorporated part of the 
prostomium, for which the continuity of the medullary and the 
cerebral folds is an argument. In the latter case the neu- 
ropore of the Craniates would be at least more comparable 
