Summary In the first part of this paper the main colour markings of Botryllus are take into account and their behaviour during the life of the colonies is described. The colour pattern of the zooids of a given colony is made of a ground colour, due to an unknown bluish pigment, and of other superimposed pigments, the white and the orange pigments. The white pigment, a purine compound probably xantine, which looks yellowish by transmitted light and silvery by reflected light, on the dorsal surface of the zooids is either scattered or arranged into definite colour markings, the intersiphonal band and the peristomatic ring. The orange pigment, a carotenoid, which unlike blue and white pigments is completely lacking in a part of the colonies, whenever present is preferentially arranged around oral and cloacal orifices, representing another typical colour marking. All the three pigments are contained in vacuolar blood cells that make their appearance during metamorphosis of the larva; but the full expression of the intersiphonal hand and of the peristomatic ring requires some blastogenic generations, whereas the orange pigment reaches the final distribution around the siphons still in the oozooid. In the second part of the paper the genetic analysis of the three colour markings, orange pigment, intersiphonal band and peristomatic ring is dealt with. All the above characters are simple mendelian characters, an allele « presence » being dominant over the allele « absence ». The characters are inherited independently. A given colony, which does not possess orange pigment on its own, can be given it by another colony with whom has fused and maintains the pigment in all the succeeding blastogenic generations; but obviously the pigment does not pass in the progeny by sexual way. In the last part of the paper the present results are put into relation with previous researches and opinions on the pigmentation of Botryllus. The various combinations of the characters here studied and minor variations in ground colour result in different colour patterns, which once were considered specific characters. This was demonstrated to he erroneous by Bancroft and the present results support his conclusion. At the same time the colour patterns, for which a genetic determination has been here demonstrated, seem to represent a valuable tool of genetic analysis of the natural populations of the species.
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