54 THE ANCESTRY OF VERTEBRATES 



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 I will try to demonstrate below (cf. anon), 

 probably corresponds to . the place of the future mouth. 

 By this process the praecerebral 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 fo f/za^o/ 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 



