neal: nervous system in squalus acanthias. 159 



ral plate iu Figure 2 do not admit of a satisfactory comparison, neither 

 is it possible, even with a prejudice in favor of finding uniform condi- 

 tions, to state exactly which segments of Figure 3 correspond to those 

 of Figure 2. 



An examination of many embryos (more than fifty) in this stage of 

 development — at which, in agreement with Locy, I have found that 

 the segments are more clearly marked than at any other stage — has 

 served only to establish the opinion that there is no constancy in their 

 number in different individuals, nor agreement in number or position 

 upon the two sides of the plate of a single individual. After an ex- 

 amination of a large number of embryos at this and closely related 

 stages, I have been compelled to abandon my first opinion, which was 

 based chiefly on the study of the embryos of Figures 1 and 2, and was 

 favorable to Locy's contention. In no case that I have seen do the 

 segments appear symmetrical, and in no case have I been able to de- 

 termine a definite relation with the somites. 



d. Continuity of the " Segments." 

 My observations have of course not been confined to this most favor- 

 able stage. While the evidence given above, based on the study of 

 embryos at a stage when the segments are most plainly seen, appears 

 to my mind satisfactory proof that true segments do not exist at this 

 time, the study of embryos in both earlier and later stages shows that 

 even these segments are only transitory structures. This evidence, 

 though in a sense negative, is not without weight in the treatment of 

 the question. It constitutes, it is true, neither proof nor disproof of the 

 genuineness of the segments. It is, however, what we should expect, 

 if we find the segments unlike iu number and size on the two sides 

 of the same embiyo and in different embryos of the same stage. A 

 want of continuity in successive stages is not, however, w'hat we should 

 expect if we were dealing with true segments. These structures would 

 certainly have much less morphological value than is assigned to them 

 by Locy, were they simply transitory and without definite relation to 

 organs which appear in later stages. Locy believes that he has traced 

 them " up to the time when they form neuromeres," but he by no means 

 makes it clear how structures which appear ".like beads " along the edges 

 of the neural plate become transformed into ventral structures such as, 

 according to his own account, the " neuromeres " are. " In the triink 

 region," he says ('95, p. 516), " the lines of division may be traced inwards 

 toward the median furrow. This is probably due to the appearance of 



