NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 159 
ral plate in 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 in number and size on the two sides 
of the same embryo and in different embryos of the same stage. A 
want of continuity in successive stages is not, however, what 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 trunk 
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 
