ASEXUAL REPRODUCTION IN THE COLONIAL TUNICATE, BOTRYLLUS SCHLOSSERI (PALLAS) SAVIGNY, WITH SPECIAL REFERENCE TO THE DEVELOPMENTAL HISTORY OF INTERSIPHONAL BANDS OF PIGMENT CELLS

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Abstract
Zooids of many Botryllus colonies develop intersiphonal bands composed of special light-reflecting pigment cells; collectively these bands constitute the intersiphonal pattern (or patterns) of the colony. A specific pattern exists for only a brief period. It is then destroyed and is replaced by an entirely different pattern. This progressive remodeling of intersiphonal patterns is intimately associated with changes during asexual reproduction. Formation and subsequent destruction of intersiphonal bands are described for the oozooid and first 7 generations of blastozooids in living colonies established from isolated larvae. An interval of 4-5 days occurs between maximum development of one pattern and that of the next. During this interval changes occur as follows: a) zooids become highly sensitive and contractile and intersiphonal patterns become very irregular; b) zooids undergo maximum and permanent contraction, their intersiphonal bands disintegrate, and the pigment cells are released into the circulatory system and are carried passively by the blood stream; c) simultaneously new generation zooids enlarge and new intersiphonal bands form in them; these are solid bands at first, but tend to become double, especially in older generations; d) reflecting cells disappear from the circulatory system. There is no evidence that pigment cells released from degenerating bands participate in the formation of new bands in next generation zooids. If an intersiphonal band appears in the oozooid, it appears in all later generations of zooids; if it is lacking, it never appears in later generations. Presence or absence of intersiphonal bands is probably genetically determined. Intersiphonal patterns vary when different colonies are compared. Possible factors involved in such variations are considered. Chief among these is the variable number of zooids in a given generation. This may be due to: differences in budding rate in different colonies, especially in the establishment of 2d generation zooids; premature atrophy of developing buds; persistence of parent zooids after their buds attain the adult state; fusion of adjacent colonies. The geometrical form of the pattern varies in a definite manner according to the generation of zooids and the number of zooids involved. The method of establishing > 1 intersiphonal pattern per colony is described. There appear to be no inherent differences between colonies in the number of zooids which participate in the formation of one intersiphonal pattern.