NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 243 



it is evident that the chief support for the hypothesis consists, first, in an 

 apparent want of a definite relation of the nerves to the neuromei'es in 

 the different Vertebrate groups, — i. e. an apparent inconstancy in the 

 structures themselves, — and, secondly, in the fact that the hypothesis 

 seems to explain the structural conditions presented. 



I turn now to a consideration of the arguments supporting the view 

 that the neuromeres are of morphological (phylogenetic) importance. 



h. Phylogenetic Interpretation. 



A phylogenetic interpretation of the foldings of the medulla was first 

 given in 1874 by Foster and Balfour. The following year Dohrn 

 accepted this explanation. Beraneck ('84) showed that in the Lizard 

 the hindbrain folds ("replis") were definitely related to certain nerves. 

 Having later ('87) confirmed his observations by studies of chick em- 

 bryos, he concluded that the foldings are the last indisputable renDiants 

 of the primitive segmentation of the head. It is notable that he reached 

 this conclusion notwithstanding the fact that, in his opinion, the seg- 

 ments of the spinal cord do not have the same characters as those found 

 in the foldings of the hindbrain. Subsequent investigators, however, 

 have sought to compare encephalomeres with myelomeres. In 1885 

 Rabl found in chick embryos a regular folding of the side walls of the 

 myelencephalon, the segments of which showed the same characteristics 

 as the foldings in the region of the spinal cord. During the same year 

 Kupff"er ('86), in studies on different Vertebrate embryos, found that the 

 foldings extended into the midbrain region. Because of the relatively 

 late appearance of the folds, — " after the closure of the neural tube, 

 after the formation of three brain vesicles, and long after the segmenta- 

 tion of the mesoderm," — Kupffer thought that there was much against 

 the interpretation of these folds as remnants of a primary general metam- 

 erism of the neural tube, but his later observations — previously cited 

 in another connection (p. 174) on an embryo of Salamaudra atra at a 

 stage before the closure of the neural plate — led him to believe that in 

 this particular case there is a primary segmentation. 



The fact that Kupffer here found eight cross furrows in the brain 

 region, representing as many " ancestral segments," appears to have 

 strongly influenced his subsequent interpretations of the morphology of 

 the forebrain in diff'erent Vertebrates, for in his later studies he has sought 

 to find evidence of these eight primary " encephalomeres " in the fore- 

 brain and midbrain, even " after the closure of the neural tube, and the 



