neal: neevous system in squalus acanthias. 161 



of the neural folds at this point (the angulus terminalis). Their late 

 appearance is possibly also to be correlated with the late appearance of 

 the anterior portion of the neural crest. 



The evidence which I have given leads me to conclude that the so 

 called neural segments cannot be traced into the "neuromeres" of 

 later stages. Of the accuracy with which Locy has traced them I shall 

 have more to say, when I speak of the limits of the cephalic plate. 



e. Interpretation of the Evidence. 



Locy interprets the " neural segments," as has already been stated, 

 as " survivals of a primitively segmented condition of the body." In 

 search for evidence to support this phylogenetic interpretation, he has 

 studied the early stages of the Torpedo, Amphibia is, and the chick. 



Torpedo embryos (p. 531) are found to be "not so favorable for the 

 study of the segments as Acanthias," yet " the number [of folds] in a 

 given region in Torpedo corresponds to that in Acanthias." In the three 

 Amphibian forms which Locy has studied (Amblystoma, Diemyctylus, and 

 Rana) "there are ahout ten pairs of segments in the broadly expanded 

 neural folds of the head." In the chick, "there are eleven segments 

 in front of the first formed protovertebrte." Locy has also found (p. .539) 

 that in the chick " the walls of the primitive groove are also divided into 

 segments that are similar to those that appear in the neural folds." ^ 



Owing to the evidence stated above, I am unable to regard the seg- 

 ments in S. acanthias as of phylogenetic value. Are they then arti- 

 facts, as suggested by Eycleshymer ? ^ I do not think so. Several of 

 the best fixing agents have been used by Locy and myself, and he has 

 in addition observed these structures in living embryos. It is known, 

 however, that different fixing agents cause differences in internal and 

 external conditions, as the result of swelling or contraction. They may 

 have served to intensify the distinctness of Locy's segments, yet it is 

 hardly probable that they are the sole cause of them. 



I believe that the segments are the results of unequal growth 

 along the margin of the neural plate. It is obviously not necessary to 



1 Italics mv own. 



2 Eyeleshymer's ('95, p. 394) observations on Amblystoma do not agree with 

 those of Locy. Eycleshymer states that "certain markings which might be in- 

 terpreted as neuromeres are often observed in the neural folds, yet their arrange- 

 ment is decidedly irregular, and one is led to believe that they indicate nothing 

 more than artifacts caused by tlie killing agents." I have carefully examined 

 Amblystoma embryos, at a stage when the neural folds are widely open, and my 

 observations agree with those of Eycleshymer. 



VOL. XXXI. — KO. 7. 2 



