FORM REGULATION IN ZOÖTHAMNIUM ALTERNANS

¹ From Bard College, Columbia University

1. Zoöthamnium alternans is a colonial protozoan of a rather special type whose constituent cells collectively possess in some degree many of the attributes of an integrated organism. Some of the integrating factors can be described in general terms from the work undertaken on form regulation.

2. When the apical cell of the primary axis is dissected away from a developing colony, a cell on some inferior branch, usually the first below the cut, will differentiate into a new apical cell. The geographical limits within which positive regulative responses occur are given in the text.

3. Development of a colony continues from the newly differentiated apical cell. The structural and developmental characteristics of the normal colony persevere in the regenerated portion.

4. Evidence is presented to the effect that zoöids retain, for a time at least, greater developmental potentialities than are actually expressed when they comprise a part of the intact colony.

5. Under varying physiological conditions in the apical region of a colony, the coördinating influences exerted upon the mitotic activity of neighboring zoöids may be inhibitory (as shown by the responses evoked after decapitation) or excitatory (when the terminal macrozoöid is transformed into an ex-conjugant).

6. In the light of observations presented, the idea of dichotomous segregation or sifting out of potencies at fission is inadequate as an explanation of localization in this species. The experimental data do not confirm Fauré-Fremiet's cytological account of qualitatively differential divisions at specified division nodes on the branches.

7. There is cause to suspect that morphogenetic processes in particular zoöids of Zoöthamnium alternans (e.g. the presumptive ciliospores), once initiated and partly expressed in visible structure, can be conditioned or modified by cuts made in some neighboring region.

8. An hypothesis is offered to account for the origin of a regenerate from one or the other of several dissimilar cells of a branch strain. The explanation is based upon the factor of time in relation to the balance between extrinsic influences and the aggregate of intracellular metabolic activities by which potentialities are realized. The cells are thought to be more susceptible to external control during the reorganizational period of mitosis. There may be a critical time in cellular differentiation beyond which the intrinsic processes are not influenced by stimuli arising in some other part of the colony.