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PAUL BRIEN
FIG. 1. Diagram of a budding hydra, an example of an animal in continuous
growth, ceaselessly repairing the wear of its tissues, and as a result, immortal as to
the maintenance of its specific characteristics. Arrows indicate the direction of the
growth participating in elongation of tentacles and emission of buds which are
detached progressively, in the end compensating for the necrosis of cells at the level
of the pedal disc.
zone where they have just appeared, toward the peduncle, as if they were
on a spiral escalator. The appearance of the buds and their downward
movement as they develop consume the continuous growth of the hydra
and contribute to the maintenance of a constant specific size (Brien,
1949).
Formation of the beautiful colonies of phylactolaemates furnishes
another example of this kind of growth (Figs. 2 and 3). The zooecia, the
individuals which compose the colony, possess, under the peristomial fold,
a ventral zone of the wall where polypidian buds appear and around
which daughter zooecia are formed. This is a definitely limited region
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