134 H. V. NEAL 



mental value. Eycleshymer ('95) also finds only a few large 

 segments of no metameric significance. Locy ('95) found four 

 or five in the region and agreed with Froriep and Eycleshymer 

 that they were of no segmental importance. Griggs ('10) recog- 

 nized one neuromere in the forebrain region, two in the mid- 

 brain and one or more in the hindbrain region and regards these 

 as the only true neuromeres. On the other hand, there are "in 

 the closed neural tube of Amblystoma a series of swellings, ex- 

 tending from the anterior end of the brain to the otic pit, but 

 since these divisions are of varying morphological significance 

 they cannot rightly be called neuromeres." By what criteria 

 shall the real neuromeres be determined? Is Griggs correct in 

 denying neuromeric value to the forebrain or midbrain vesicles 

 or their secondary subdivisions? Are the true neuromeres those 

 of the open neural plate of Amphibia, as Griggs maintains, or 

 are the real neuromeres those of the closed tube as held by the 

 majority of morphologists? The difficulty of determining the 

 real metamerism of the nervous system is increased, if morpho- 

 logical opinions are influenced at all by the segmentation of the 

 brain of Bdellostoma. 



Dr. Bashford Dean, in a letter which he kindly permits me 

 to use, makes the following statement of the neuromeric con- 

 ditions in Bdellostoma embryos: 



If we believe that a 'neuromere' is represented objectively by a spe- 

 cially and definitely dilated spot in the central nervous system, expressed 

 either in the inner lumen or in the outer wall of the medullary axis, 

 we certainly cannot interpret the conditions in the hag-fish in terms, 

 for example, of the shark or the amphibian. In the first place, the 

 hag-fish embryos may show a great number of these dilated regions, 

 as many indeed as twenty-seven or twenty-eight in the midbrain 

 and hindbrain. They may indicate also that these dilated areas 

 are indefinite in number; and that in relatively the same age these areas 

 may be either very obscure, absent, or barely visible, or may be reck- 

 oned with almost mathematical precision. They are moreover, rarely 

 symmetrical; it is curious also that their symmetry is never expressed 

 in the same way in the large series of embryos examined. Neuromeres 

 are difficult to distinguish in the forebrain of Bdellostoma. Occasion- 

 ally there appear to be two, three, or four present. These can be dis- 

 cerned faintly on one side or the other side of the medullary axis, 

 never paired. 



