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 neuromeres 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. 
b. PHYLOGENETIO 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. “Béraneck (’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 remnants 
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 
Kupffer (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 Salamandra 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 different 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 
