34 PROF, J. ARTHUR THOMSON ON 
REGENERATION OF LOST PARTS IN ANIMALS. 
By J. ArrHur Tomson, M.A., F.R.S.E., Professor of 
Natural History, University of Aberdeen, Vice-President. 
(Read 14th February 1901.) 
In the eighteenth century the attention of naturalists was 
for a time focussed on the problem of regeneration or the 
regrowth of lost parts. Trembley discovered to his delight 
that the freshwater polyp—AHydra—might be multiplied 
by being cut in pieces ; Spallanzani showed that the earth- 
worm cut by the spade might regrow a new tail or even 
a new head; Bonnet made numerous experiments on other 
worms and thought out an elaborate theory; Réaumur 
pointed out the advantage of the regenerative capacity in 
animals which were in natural conditions exposed to frequent 
risk of breakage or wounds. Neither facts nor interpreta- 
tions were awanting a hundred years ago, and yet the 
problem has come down to us still unsolved. 
Of recent years, however, some progress has been made, 
The remarkable development of experimental embryology, 
and the happily growing tendency to test biological 
hypotheses by experiment, have prompted fresh work on 
the subject of regrowth. The basis of fact has been greatly 
broadened, and perhaps it is not too sanguine to say that 
the interpretations have become a little less vague. The 
literature of the subject is already so enormous that it 
suggests the usefulness of attempting a sort of balance- 
sheet of fact and opinion. In so doing, I have confined 
myself to adult animals, leaving plants and young embryos, 
not out of account, but undiscussed. I am especially 
indebted to recent papers by Weismann, Morgan, and 
Barfurth. 
What I mean to discuss is really only one aspect of 
the regrowth of lost parts in adult animals, but it is 
advisable to recall a number of analogous phenomena. 
One of the first two or even four cells into which an egg 
