neal: nervous system in squalus acanthias. 167 



English equivalent of Kupffer's " Medullarfalten " and of Beraneck's 

 "replis medullaires" (medullary folds) is used with an entirely diflferent 

 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) mydomeres, due to con- 

 strictions of the myeloa ; (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 beghi 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 arc of 

 a circle." (2) "The consti'ictions 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 neui'omere." Later investigations have served only 

 to confirm this clear analysis of the structure of a neuromere. 



h. Development of Hindbrain Neuromeres. 

 Previous investigators have assumed that the hindbrain neuromeres 

 possess the same characteristics at their first appearance that they do in 



