380 
mination of many embryos (more than fifty) at this most favorable 
stage leads me however to conclude that there is no constancy 
in the number of “segments” in different individuals 
and no agreement in number or position upon the two 
sides of the neural plate of a given individual. After a 
careful examination of a large number of embryos in this and very 
closely related stages (comprising more than two hundred and fifty 
individuals between BaLrour’s stages C and F), I have been com- 
pelled to abandon my earlier opinion, which 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 establish a definite 
relation with the somites. Locy believes that he has traced the 
“neural segments” up to the time they form “neuromeres” but he 
by no means makes it clear how structures which, as he states, appear 
like beads along the edges of the neural plate become transformed 
into ventral structures such as, according to his own accomit, the 
“neuromeres” are. I have also found it impossible, as would be ex- 
pected from the evidence of irregularity and the want of symmetry, 
to trace definite “segments” through different stages. Locy affirms 
a shifting of relations of his neural segments. The difficulty of 
tracing “delicate structures” like the “neural segments” through 
successive stages, while they are shifting in position, is obvious. 
EYCLESHYMER (795, p. 394) has observed in Amblystoma the 
structures described by Locy as “neural segments”. He finds their 
arrangement decidedly irregular, and is led to believe that “they 
indicate nothing more than artifacts caused by the killing agents”. 
My own observations on Amblystoma agree with those of EYCLESHYMER 
regarding the irregularity of the segmentation. But while these 
structures may be intensified by killing agents, I am unable to 
regard them as artifacts. Locy’s evidence of their normal existence 
I regard as quite satisfactory. I believe that the “segments” along 
the edge of the neural plates are the result of unequal growth. But 
it is obviously not neccessary to regard irregularities on the edge of 
a rapidly expanding plate of tissue as of deep morphological importance. 
The “segments” in the cephalic region show most prominently just 
before the edges of the plate begin to rise dorsally. It is a fact of 
great significance that the first evidence of the disassociation of cells 
to form the neural plate appears at precisely this stage. Such a dis- 
association of cells along the edges of the neural plate, or even a 
rapid proliferation of cells in this region, might lead to the appearance of 
segmentation. An examination of cross sections of the cephalic plate 
