NEAL : NERVOUS SYSTEM IN SQUALUS ACANTHI AS. 179 



Kupffer finds at least jive eucephalomeres in the primary forebrain. 

 This conclusion seems strengthened by the conclusion of Burckhardt ('93), 

 that the median zones of the neural tube retain throughout the Verte- 

 brate series the primitive segmentation best, and therefore are the best 

 for comparison. 



My conclusions from a study of the evidence presented by those who 

 have assumed a segmental value for the secondary subdivisions of the 

 forebrain and midbrain vesicles are, (1) that morphologically different 

 structures have been described by them as " neuromeres " or " eucepha- 

 lomeres," and (2) that the divergence in their results does not seem to 

 justify this assumption. 



I now turn to an examination of the development of forebrain and 

 midbrain regions in S. acanthias, in order to determine whether or not 

 it is probable that structures morphologically comparable with hindbrain 

 neuromeres exist in these regions. Since hindbrain neuromeres involve 

 all three zones — dorsal, ventral, and lateral — of the walls of the 

 encephalon, the value of forebrain and midbrain segments as morphologi- 

 cal equivalents of them will clearly depend on their similarly involving 

 those zones. If they do not, it is incumbent upon one who holds to 

 their equivalency to demonstrate how modification has probably obscured 

 or obliterated the primitive conditions. Evidence in such a highly 

 specialized region can be at best only probable. Here, however, as 

 always, the demonstration of morphological comparability must be 

 " controlled " by the demonstration of similar relationships to other 

 organ systems. 



h. Development of the Forebrain and Midbrain. 



At a stage with 19 or 20 somites the conditions in the anterior brain 

 region are very simple. The primary forebrain and midbrain are simple 

 vesicles or enlargements of the neural tube. A parasagittal section cut 

 through the right wall of the neural tube is represented in Figure 45, 

 Plate 7. Six vesicles are counted, all of them being included in the 

 region of the cephalic plate. The anterior vesicle shown is the wall of 

 the forebrain in the region of the optic vesicles. Behind lies the mid- 

 brain, separated by a slight constriction from that region of the hind- 

 brain to which Zimmermann ('91) has given the name " Hinterhirn." ^ 

 Hindbrain neuromeres IV, V, and VI are clearly defined. 



A frontal section of an embryo of the same stage, so cut as to coincide 



1 The English term hindbrain has been applied to the region separated bj 

 the Germans into " Hinterhirn " and " Nachhirn." 



