158 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
region, are not equally distinct, it being very difficult, if not impossible, 
to determine the boundaries of some of them. They also differ consider- 
ably in length and apparently without any regularity, a condition not 
easily reconciled with the interpretation of them as true segments. It 
would certainly be impossible even in this specimen to point out with 
certainty corresponding segments on opposite sides of the cephalic plate. 
In the trunk region of the same specimen no correspondence between 
somites and “neural segments” is seen. However, a faint lobing of 
the inner margin of the tail fold is seen on the right side of the embryo. 
Locy’s (95, p. 519, Fig. 29) description of a stage as close to this as 
any figured by him is as follows: “They [the segments] appear like a 
row of beads running along the ventrally recurved margin, and extend 
with great distinctness the entire length of the embryo. Those in the 
trunk region are continuous with those of the head, and pass into the 
latter without any transition forms. There is, however, some individual 
variation in size of the neuromeres, and they are not absolutely sym- 
metrical on the right and left sides, but the significant thing is, [that] there 
is uniformly the same number on each side in a given region, such as the 
hindbrain, or the brain region as a whole. . . . There seems now to be 
a natural landmark separating the ‘cephalic plato” from the rest of the 
embryo ; this is an abrupt downward bending in the medullary folds, 
which, as I have determined, lies just in front of the future origin of 
the vagus nerve. There are eleven metameres* in the lateral margins 
of the cephalic plate, including the ones embraced in this fold.” The 
accuracy of this conclusion I shall discuss in treating of the question of 
the limit of the cephalic plate (p. 162). I wish here only to call atten- 
tion to the fact that none of the reproductions of Locy’s photographs, 
with two possible exceptions (his Figs. 2 and 23), show a segmentation 
of the neural folds in either the trunk or the embryonic rim. 
If now we turn to Figure 3 (Plate 1), we find an embryo of about 
the same stage as that shown in Figures 1 and 2; at loast, it has the 
same number of somites (6 to 6%). The conditions are these. The “ seg- 
ments” at the margin of the neural plate differ markedly in. distinct- 
ness, and are irregular in size. In the region of the cephalic plate — 
the posterior boundary of which is marked by the arrow — the number 
of segments on the right and left sides is not the same, I was not 
able to assert this with so much confidence in regard to the embryo of 
Figures 1 and 2, since in that embryo the limits of the cephalic plate 
were less clearly defined. If the segments of the two sides of the neu- 
1 Italics mine. 
