1. The first division of the initial macrozooid (or ciliospore) determines the median antero-posterior plane of the colony; the subsequent cleavages of the daughter individuals are brought about according to equally determined schemes, which give the main strain (or axial trunk) and the lateral branches, alternately at right and at left.2. The individuals constituting the main strain are of a rather large size (axial macrozooids); their cleavage is always accompanied by a differential division giving rise to a new axial macrozooid and a median microzooid.3. The differential divisions are characterized by an unequal division of the protoplasmic mass, accompanied either by a sensibly equal division of the macronucleus (division supposed to be quantitatively differential), or by the unequal division of the macronucleus in which the larger mass (delicately granular) remains in the larger individual, while the thinner part (often of fibrillar structure) goes to the microzooid (division supposed to be qualitatively differential).4. The cleavages of the ciliospores and those of the axial macrozooids, I, II, and III are always differential as regards the protoplasm and the nucleus. The cleavages of the macrozooids IV and after give a cytoplasmic differential division and an equal nuclear division; the differential division of the macronucleus is carried back to the cleavage of the corresponding median microzooids.5. The common microzooids have a limited power of growth and of multiplication.6. The median individuals having a large macronucleus after the differential division of the median microzooids D and progeny begin an active period of growth accompanied or unaccompanied by only one ulterior division: these forms constitute the median macrozooids or "ciliospores."7. The growth of the ciliospores is accompanied by an important hypertrophy of the macronucleus followed at first by a disintegration, then by a reconstitution through an endomictic process.8. During the growth of the median macrozooids, some grains of secretion accumulate at the individual's posterior end, then the ciliary crown grows, the ciliospore breaks away, swims freely, then settles down on a substratum and becomes the source of a new colony.9. The character of the differential divisions on the main strain seems to determine the individual's differentiation of the colony; this differentiation depends not only on the individual's size, but also on its physiological potencies.10. Independently of the obviously differential divisions, it is shown that the power of growth is divided among the microzooids according to a gradient, so to speak.11. The unequal power of growth of the various individuals of a colony gives to its whole growth a behavior which approaches the behavior of an organism. This unequal share constitutes for the growth of the whole a limiting factor very unlike a factor of senescence.