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Abstract

The first part of this study describes the normal pattern in terms of which budding takes place as a colony composed of a large number of zooids is generated from an oozooid. During the time interval which elapses from the formation of a zooid until it reproduces asexually by strobilating, size increase is logarithmic. The length of time which elapses until a given zooid strobilates is correlated with the size of its thoracic region and its prior history.

The role of zooid size increase in bringing about the conditions which lead to the initiation of strobilization was examined by irradiating small zooids which had been growing for a short period of time with different dosages of gamma radiation. The dosages of irradiation used slowed down the rate of growth or stopped growth; however, these animals strobilated at the same time that the unirradiated controls strobilated, demonstrating that the initiation of strobilization is not tied to the growth cycle of the zooid.

The possible role that organ systems situated in different regions of these animals might have in initiating strobilization was studied by removing parts of zooids at different times during their growth cycle. These experiments demonstrated that a population of cells in the dorsal half of the thorax normally function to inhibit strobilization. Studies in which parts of animals were made to regenerate thereby creating a condition in which different regions of the same zooid were of two different chronological ages demonstrated that the activities of cells in the thoracic region of these animals also regulate the time interval which will elapse until strobilization. These experiments were done by compounding “Siamese twin” zooids with two thoracic regions and showing that the youngest thoracic region controls the timing of strobilization.

This work is discussed in relation to experimental work which has been done on other ascidian species and to the suggestions that have been made for the purpose of explaining how asexual reproduction is regulated in tunicates.