44 PROF. J. ARTHUR THOMSON ON 
areas liable to injury have lagged a little behind the evolu- 
tion of the part to which they correspond, and which they 
are able to replace. I should be more inclined to restate 
the facts more vaguely, but perhaps more cautiously—that 
residual powers of embryonic growth are here and there 
persistent, ready to express themselves on appropriate 
stimulus, but tending to a simpler result than that attained 
in the course of the normal slow growth, partly because the 
regeneration tends to be rapid, and partly because the con- 
ditions are different, 
A crab loses a limb, the wound is closed by the cuticle, 
and a limb-bud begins to appear. Whether we imagine that 
there are special regeneration-germs specifically constituted 
for limb-making and resident at likely spots, or suppose that 
there is in the bud, as in the imaginal discs of a pupating 
insect, a return to embryonic conditions, a re-arrangement 
of material under the influence of novel stimuli, or make 
other suppositions, we have to face the fact that the re- 
generation does not always come up to the mark. 
An antenna may grow in place of an _ insect’s leg 
(Wheeler), or a leg instead of an antenna (Kraatz). An 
abdominal appendage in the edible crab may be replaced by 
a walking leg (Bethe), or an antenna instead of the eye- 
bearing stalk ina shrimp. The cases where a Crustacean 
does not get an eye for an eye, but something simpler, have 
been especially studied, and are very interesting (Herbst 
and Morgan). In Palewmon, Herbst observed that a lost eye 
was replaced by a short stalk-like structure with sete and 
a flagellum; in Sieyona, the new part had sometimes a basal 
piece and two branches (Archiv Entwicklungsmechanik, 
li, (1896), pp. 544-558 ; Festschrift Nat. Ges. Ziirich, ii. pp. 
435-454, 1 pl.). 
We need not multiply instances, but it should be noticed 
that most of them concern animals whose limbs normally 
pass from one form to another with successive moultings, 
and, as Przibram suggests, it is worth asking whether the 
antenna instead of an eye was really the final result of the 
restorative process. In his experiments on Daphnia and 
Asellus, he found that the limb which came from a regenerat- 
ing limb-bud was not at first perfect, though it became 
