DEDIFFERENTIATION IN PEROPHORA 675 
dominance. In Stenostoma, fission occurs according to a regular 
system, so that the relative age of each head-region in a chain 
can be determined. If now a cut is made so that a younger 
zooid is left in front of an older zooid at the anterior end of 
a piece, this younger zooid, though morphologically complete, 
will be resorbed by the posterior. If completely isolated from 
the posterior zooid the younger one would have been capable 
of leading an independent and normal existence, so that 
the age-relation of zooids clearly determines dominance. 
During the process, ‘ disintegration ’ (presumably migration 
of the cells from the tissues) of the subdermal structures 
occurs, and the pseudoccel becomes filled with cells and 
granules. The posterior undestroyed zooid grows more rapidly 
than usual, apparently because of the excess of nutriment thus 
provided (although this nutriment is in the pseudocoel and not 
in the gut). Child did not undertake a histological examina- 
tion. From his observations in vivo, however, it is clear 
that the cells migrate out of the tissues, as in Perophora. 
The most highly-differentiated organ, the pharynx, disinte- 
grates very early. The intestine, however, does not do so until 
late. From the ectoderm of the resorbed portion a very 
gradual migration probably occurs. The portions undergoing 
resorption are wrinkled and collapsed. 
The reversibility of the process is shown by the following 
observations. If an older posterior zooid has in front of it 
another almost as old, resorption will begin, but fission will 
oceur before it has finished, and the two zooids will separate ; 
after this the anterior zooid redifferentiates. The converse 
of this is seen when a long anterior fragment is present. In 
this case the beginning of regeneration occurs, but reduction 
finally takes the upper hand, and the whole fragment is 
resorbed. The rapidity of the change is noteworthy, complete 
resorption usually occurring in twenty to thirty hours. Pro- 
vided that the brain-region ef an anterior fragment is absent, 
resorption will occur ; even when a system consists of a very 
long but brainless anterior fragment, and only the brain-region 
of the posterior zooid, resorption happens. 
