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1. The frond-like colonies of Zoöthamnium alternans develop according to a well-determined pattern. This development is described with particular reference to the origin of the heteromorphic zoöids.

2. An asexual propagative zoöid (ciliospore) detaches from a mother colony, swims away, and settles down to form a new colony. When affixed to the substrate the ciliospore begins to secrete a peduncle or stalk. The first portion of the peduncle is flexible but not contractile. From a point some 200 to 300µ above the hold-fast, the ciliospore suddenly begins to produce a contractile cord in the core of the hyaline stalk substance. Distal to this point the contractile cord is an integral part of the stalk; it is continuous from branch to branch and from cell to cell.

3. When the stalk is approximately 500µ long the ciliospore divides unequally. The larger daughter remains axial in position. It represents the first generation of the terminal macrozoöid series (TM. No. 1). The smaller lateral cell is the initial zoöid of the first branch, the median microzoöid A.

4. Successive divisions of the terminal macrozoöid produce each time a terminal macrozoöid of the next generation and a median microzoöid. The latter are strictly alternate in position: they alternate on right and left sides of the primary axis at successive nodes. As many as thirty-three generations of the terminal macrozoöid have been observed.

5. The first division of the initial branch zoöid gives rise to an axial microzoöid and a lateral stem cell. The stem cell generates alternating zoöids of the main branch strain whereas the axial microzoöids on some of the branches represent the presumptive ciliospores.

6. At least four types of zoöids comprise a colony: (1) a single terminal macrozoöid at the apex of the primary axis, (2) many common microzoöids scattered along each branch axis, (3) a terminal zoöid at the tip of each branch, and (4) immature ciliospores which arise from axial microzoöids on some of the branches. During epidemics of conjugation two more types may be formed: the terminal macrozoöid may be transformed into a macrogamont, and common microzoöids at random positions may metamorphose into migratory microgamonts. The spatial distribution of the heteromorphic zoöids is described in some detail.