36 ; PROF. J. ARTHUR THOMSON ON 
In other words, a potency which, to begin with, is only a. 
subsidiary alternative, may in special circumstances attain 
an essential predominance. This, also, should be borne im 
mind when we think of regenerative phenomena. 
Fourthly, it may be profitable to keep by themselves 
those cases where an artificially cut off small fragment is 
able to reproduce an entire animal. In regard to the 
Protozoa, we know that a fragment of a cell may in 
propitious conditions regrow an entire creature, provided 
always that the fragment be in some measure nucleated— 
provided, that is to say, that the organisation of the cell- 
firm is not destroyed by the fragmentation. The experi- 
ments on Stentor, for instance, illustrate this point clearly. 
There does not appear to be any special problem here, since 
so many of the Protozoa are in the habit of multiplying by 
fission, or by giving off buds. 
Similarly, in the familiar case of Hydra we know that 
a fragment cut off will regenerate an entire polyp, provided 
that it is not too minute in size (a quantitative limit), and 
provided that it is a fair sample of the body and not, for 
instance, a mere tip of a tentacle (a qualitative limit). 
There does not appear to be any special problem here, since 
the continuance of Hydra is mainly dependent on its. 
multiplication by buds. And the same might be said of 
many Planarians, which also multiply asexually. 
We see, then, that in approaching the problem of the 
regeneration of lizards’ tails, newts’ legs, snails’ horns, 
starfishes’ arms, it is useful to bear in mind :—(1) the facts- 
of experimental embryology, (2) the normal process of 
tissue-regeneration, (3) the frequency of reproduction by 
buds, and (4) those cases where a mere fragment regrows: 
the whole. 
The first fact in regard to the regrowth of lost parts 
which I wish to illustrate is that the capacity has a very 
unequal distribution among animals. It is very common in 
worm-types, but almost absent, I believe, in Nematodes ; it is. 
common in Cheetopods, but unknown in leeches; it is general 
among Arthropods, but there is not very much of it in 
Molluscs ; it is well seen in Amphibians, especially in the 
tadpoles and newts, but it is not much in evidence among 
