No. I.] VARIATIONS IN LIMULUS POLVPHEMUS. 49 
that have arisen in attempting to explain, first, the increase in 
length at the posterior end, and second, the relation of the 
primitive streak to the concresced lips of the ‘“blastopore.” 
In fact, the whole manner of interpreting the formation of 
germ layers in vertebrates may be advantageously reconsidered - 
from this new standpoint. 
I can only touch on these points here, but I shall discuss 
them more fully elsewhere. The figures are too suggestive to 
be passed over without comment. 
That a half-metamere is to be regarded as the unit of structure 
of segmented animals is shown, above all (1),as already indicated, 
by their method of growth and direction of specialization, and (2), 
by the manner in which half-metameres are omitted or increased 
or diminished in size, rather than whole ones. On this supposi- 
tion, certain imperfections in the segments of otherwise typi- 
cally segmented animals can be explained ; such, for example, as 
spiral segments, which may be accounted for as due in part to 
the imperfect union of twin half-metameres on the dorsal side 
of the body. Instead of meeting each other squarely, and thus 
forming a line of equalization of forces, they have grown past 
each other in opposite directions, and thus each forms a more 
or less perfect spiral ! 
Polarity and embryonic axes thus appear in a new light. 
In any segmented animal the embryonic half-metameres group 
themselves in such a way that they may be represented by 
axes meeting at a shifting point, as shown in Fig. 5. The line 
H.. T. represents the median line and the axis of longitudinal 
growth, or of repetition, and the triangle ad 7, the form that 
would result if the individual half-metameres did not mutually 
modify each other. 
Wx. and p.x. represent the axes of specialization, along 
which new organs are produced in each half segment. The 
growth along these axes is at right angles to the median line, 
and also parallel with it. The tendency to a concentration of 
organs at the forward end of the embryo, or toward a median 
fusion and antero-posterior degeneration there, may be explained 
