316 JULIAN 8. HUXLEY 
(c) Size Relations; Viability. 
Wilson (loc. cit.) found that the size of the restitution- 
masses produced by Hydroids was of great importance. Large 
masses almost invariably died early, while too small masses, 
though living for a long time, failed to produce Hydranths 
or even coenosarcal outgrowths. 
In Sycon also, very small masses, though reaching a two- 
layered stage and occasionally forming spicules, fail to meta- 
morphose. Similar failure to produce normal structure from 
pieces below a certain definite size is well known in studies 
on regeneration, both in unicellular and multicellular organisms. 
It may be partly due to mere lack of material, but undoubtedly 
also, in some way not as yet properly understood, to the 
relatively greater surface and the consequences thereon atten- 
dant—differences of gaseous exchange and difference of stimula- 
tion by the environment being prominent. 
Similarly, in too large a mass, it does not appear that proper 
oxygenation for the central cells can be provided, and so 
disintegration sets in. Wilson found the interesting fact that 
successfully-metamorphosing masses were of the same order 
of size as normal planulae. The same is roughly true for Sycon, 
although here the upper limit of size for successful masses is 
much further above the larval size than m Hydroids. 
De Morgan and Drew comment on the fact that their restitu- 
tion-masses, although not metamorphosing, were much more 
resistant to laboratory conditions than the normal colonies, 
and regard it as surprising. There should be no ground for sur- 
prise in this—the cells of the restitution-masses are definitively, 
as we have seen, in a dedifferentiated condition. Experiments 
on Perophora and Obelia show that the undifferentiated stolons 
and hydrocaulus remain perfectly healthy in conditions causing 
dedifferentiation and resorption of the zooids. Clavellina 
and other Ascidians hibernate in the form of ‘ winter-buds ’, 
which are of somewhat similar nature to restitution-masses ; 
and the normal gemmules of sponges have also something 
in common with them. In the laboratory the hydriform 
