676 JULIAN 8. HUXLEY 
To sum up, whenever in Stenostoma a system is artificially 
produced in which a posterior brain-region is older than any 
brain-region anterior to itself, or has a brainless region anterior 
to it, resorption of such anterior regions will start; it will 
be completed unless fission of the system occurs during the 
process, which only happens when an anterior zooid is far- 
developed. 
A brain-region is physiologically dominant over all 
other tissues of the same and other zooids, and over all younger 
brain-regions than itself. When a region comes to lie anteriorly 
to a physiologically dominant region, it cannct maintain 
itself, and is resorbed. Antagonistic to resorption is the 
process of regeneration (morphallaxis). Both processes often 
start simultaneously in a fragment; which of the two even- 
tually gains the upper hand is determined by the age of the 
fragment. The systems resemble the stolon-zooid systems of 
Perophora, except that the different members of the system 
are all similar to each other except in age. Further, reversal 
of the effects by alterme the environment has not been 
attempted. This would provide an interesting field for 
experiment. 
As the facts stand, the dominance is caused entirely by the 
internal factor of physiological state due to (1) presence and 
(2) age of brain or brain-region, and resorption is produced when 
a part is caused to lie in an abnormal position relatively 
to a deminant region. As Child points out, similar resorption 
of parts in abnormal positions is frequently seen in erafting 
experiments in Hydra and Planarians. Subordinate portions 
in a normal position relative to a dominant region do not 
of course become resorbed. 
Once more the essential fact is that, in certain conditions, 
parts of a system are unable to maintain themselves 
of their normal size or their normal form, and, once they start 
dedifferentiating, become subordinate in the system, and can 
be used as food for the remaining dominant part. 
T suspect that investigation would show that the first change, 
here as in Perophora, is the loss of the normal cell-form of the 
