NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 167 
English equivalent of Kupffer’s “Medullarfalten” and of Béraneck's 
“ replis medullaires ” (medullary folds) is used with an entirely different 
meaning from that intended by these writers, Orr ('87, p. 335) employed 
the term “neuromere ” for the folds due to symmetrical constrictions seen 
in the hindbrain and the thalamencephalon, and distinctly stated that 
in Lizard embryos no neuromeres are found behind the vagus nerve. 
This limitation of Ahlborn’s term has not, however, been accepted by 
later investigators. McClure ('89 and ”90) again extended the term 
neuromere so as to include all the constrictions and dilatations of the 
neural tube, and classified neuromeres into : (1) myelomeres, due to con- 
strictions of the myelon; (2) encephalomeres, resulting from constric- 
tions of the encephalon. The latter term had, however, been previously 
used by Wilder (89) for the large encephalic vesicles. Zimmermann 
(91) adopted the term encephalomere, although he did not attempt to 
compare “ Encephalomeren ” with “Myelomeren,” and Froriep (’94) used 
the term for theoretically homodynamous segments of the neural tube in 
the region of the head. He stated that the encephalomeres may corre- 
spond with neuromeres, but that this correspondence is not self-evident. 
I shall adopt the nomenclature proposed by McClure (’89 and ’90). 
In my account of the segmentation of the brain I shall begin with the 
conspicuous constrictions and enlargements of the hindbrain, which have 
uniformly been regarded by morphologists as typical neuromeres or 
encephalomeres. Orr’s (87) criteria for neuromeres, based on the study 
of the hindbrain of Lizard embryos, are as follows: (1) “Each neuro- 
mere is separated from its neighbors by an external dorso-ventral con- 
striction, and opposite this an internal sharp dorso-ventral ridge, so that 
each neuromere (i. e. one lateral half of each) appears as a small aro of 
a circle.” (2) “The constrictions are exactly opposite on each side of 
the brain.” (3) “The elongated cells are placed radially to the inner 
curved surface of the neuromere.” (4) “The nuclei are generally 
nearer the outer surface, and approach the inner surface only toward the 
apex of the ridge.” (5) “On the line between the apex of the internal 
ridge and the pit of the external depression, the cells of adjoining neuro- 
meres are crowded together, though the cells of one neuromere do not 
extend into another neuromere.” Later investigations have served only 
to confirm this clear analysis of the structure of a neuromere. 
b. Devetorment or HINDBRAIN NEUROMERES. 
Previous investigators have assumed that the hindbrain neuromeres 
possess the same characteristics at their first appearance that they do in 
