INOS 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 87 
in terms of the rate. The original variation may be local or 
general, but in either case, the deficiency, if there be one, is 
not restored by nutrition, or by drafts on a general supply. 
(3 and 4). Variation in the size of separate organs or of the 
whole embryo, or the entire absence of organs, seems to be 
due to a variation in the amount of formative material of that 
particular kind out of which the variable organs are formed. 
The variation in the amount of formative material expresses 
itself in these cases in a variation in the size of the parts, not 
in their rate or range of development. The formative material 
when diminished or absent does not appear to be restored or re- 
plenished, because regeneration of defective or absent parts 
does not take place, although the organs that are present find 
plenty of material with which to continue their own growth. 
The thing then that is lacking in this case seems to have a 
definite location and is not distributed throughout the embryo. 
The deficiency is due apparently to the absence of some kind 
of formative material, and not merely to a diminution of its 
formative powers or to a variation in the quality. 
(5). Organs once formed frequently disappear by median 
fusion and degeneration, zz the reverse order of their age 
and specialization. In other words, on a given segment 
the right and left organs telescope into each other at the 
median line, and disappear one after the other in the order 
of their position and original formation. This mode of 
degeneration must be due to some inherent structural con- 
ditions, and not alone to mechanical stress or tension, because 
it always occurs in the same way at various but well-determined 
places, where the mechanical stresses due to growth of the sur- 
rounding parts must be quite different. We may assume that 
the reason the median end of a half segment disappears first is 
because it is most specialized or most highly developed, and 
therefore most likely to feel the effects of diminished vitality and 
increased tension. The other parts follow in order for the 
same reason, but the reason the latter move bodily toward the 
median line is because a path of least resistance is constantly 
reéstablished there by the degeneration of the organs nearest 
that point. 
