REGENERATION OF LOST PARTS IN ANIMALS. 35 
divides may in many cases form a complete embryo, 
sometimes dwarfed in size (Lancelet), sometimes normal 
(Sea-Urehin). One of the first two cells into which the 
frog’s egg divides will develop by itself and form, according 
to the conditions of the experiment, a half-sized embryo (if 
the injured ovum be allowed to float freely) or a one-sided 
half-embryo (if the injured ovum be kept fixed), but the 
one-sided half-embryo may at a certain stage regenerate the 
missing half. Similarly it is very easy to make a single 
ovum of bird, frog, sea-urchin, etc., form a double embryo 
instead of the normal single one. There is a large body 
of similar facts, which show this, at least, plainly—that a 
living unit has often the power of doing much more in the 
way of growth than is normally required of it. And this, 
as it seems to me, is an important general result to bear 
in mind. 
Secondly, we should recall the familiar fact that in many 
animals there is an almost continual process of tisswe- 
regeneration going on. Worn out epidermic cells, glandular 
cells, blood-corpuscles, etc., ete., are all replaced by what we 
may call a process of continued growth. We pass gradually 
from the regrowth of our hair and finger-nails to more 
complex phenomena, like the annual regrowth of stags’ 
antlers. In short, to meet the exigencies of normal life, 
there is a continuation of local growth, which has also its 
bearing on the regeneration of accidentally lost parts. 
Thirdly, it seems useful to remember that among Sponges, 
Ceelentera, Annelids, Polyzoa, and Tunicates, we find buds 
separated from the parent body to form new individuals— 
a process which leads us towards the regeneration of an 
entire animal from an artificially excised part. 
But the power of asexual reproduction, which is normally 
a mere alternative, may become the all-important power for 
the species, as is well illustrated by the case of an Alpine 
Planarian studied by Voigt. It is a relict of glacial 
conditions, a northern form, abounding in the mountain 
streams around Bonn; observation and experiment show 
that warmth hinders its sexual reproduction; out of 4000 
specimens not one was sexual; it is keeping its foothold 
in existence solely in virtue of its asexual reproduction, 
