166 bulletin: museum of comparative zoology. 



cephalomere VII, I see no escape from the conclusion that he has not 

 "traced neural segments accurately up to the time they form neuro- 

 meres." It is hardly conceivable that he will bring forward in this 

 instance the explanation previously offered in a similar case of mistaken 

 identity, that somehow, between the stage with an open neural plate and 

 a closed tube, segment 10 (neuromere VI) has insidiously come to as- 

 sume the position previously occupied by his segment 11, and that seg- 

 ment 11 has been crowded backward. And it is likewise improbable 

 that he would follow this explanation with another, — as he did in the 

 case mentioned, — that encephalomere VI of my figures represents the 

 "combined vesicle" of his segments marked 10 and 11.^ 



I now turn to the study of what I regard as the true primitive 

 segmentation of the nervous system, — the so called neuromeric seg- 

 mentation. 



II. The " Hindbrain Neiiromeres " in S. acanthias. 



a. Definition of the Term "Neuromere." 



In the preceding description the term " neural segment," or simply 

 " segment," has been used as a non-committal term for structures of 

 such different morphological value as those described by Locy under that 

 name and the regular foldings of the neural tube. Locy ('95) has used 

 the term " metamere " as synonymous apparently with his term " neural 

 segment." Since, however, the term " metamere " is applicable by usage 

 only to the successive similar parts of the body as a whole, it cannot be 

 applied wisely to the successive parts of a single organ system, such as 

 the nervous system. 



Ahlborn ('84*) was the first to use the term "neuromere," and he 

 applied it to all the successive similar segments of the central nervous 

 system. Beraneck ('84) applied the term " replis medullaires " and 

 KupiFer ('86) the term " Med ullarfalten " to the regular foldings seen in 

 the brain region of Vertebrate embryos, those of the hindbrain being 

 given by Beraneck the special appellation of " vrais replis." Since the 



1 In a paper which comes to hand just as this goes to press, Locy ('97) states 

 that he finds t\TO sets of vesicles in the brain of chick embryos. Of these tlie 

 first set, numbering seven in all, called by Locy " optic vesicles," are very ephemeral 

 in existence, and have nothing whatsoever to do with the second set, called by him 

 "brain vesicles." In Acanthias (Squalus) also he finds at least nine pairs of " optic 

 vesicles," likewise very transitory. The exact relation of these to the "meta- 

 meres " or " neuromeres " he does not state. 



