The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . with the gemmation in Clavelinidae,Pyrosomidae and Thaliacea outside this group. Secondly, thereis the parietal or peribrancliial type, seen in the Pecloso-mata, typically in the Botr^llidae. The remarkable process 01gemmation, seen in the families Didemnidae and Diplosomidae mayprobably be regarded as a modification of the stolonial type. Thedouble embryo in the Diplosomidae is probably to be interpretedas precocious budding (rather than as embryonic fission), due toacceleration in development (t
The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . with the gemmation in Clavelinidae,Pyrosomidae and Thaliacea outside this group. Secondly, thereis the parietal or peribrancliial type, seen in the Pecloso-mata, typically in the Botr^llidae. The remarkable process 01gemmation, seen in the families Didemnidae and Diplosomidae mayprobably be regarded as a modification of the stolonial type. Thedouble embryo in the Diplosomidae is probably to be interpretedas precocious budding (rather than as embryonic fission), due toacceleration in development (tachygenesis). The type of budding,and even details such as the length of the stolon, have much to dowith differences in the nature and appearance of the colonies pro-duced. The stolon, which has a wall continuous with the body-wallof the parent, contains an endodermal element in the form of theso-calleid epicardium, and also a prolongation of the ovary, or_at least a string of migrating germ-cells, so that the reproductiveelements are also handed on. Still, it is clear from recent researches. (After Pizon.) Fig. 30.—Young buds of Botryllus sectioned to show the separation of the branchial ivb) from the peribranchial {cp) cavities. ov. Dorsal tube. g/i, Germ cells. m, Mesoderm cells, ^ct. Ectoderm. that the development of the bud (bla^stozooid) and that of the embryo(oozooid) do not proceed along parallel lines. It is impossible toharmonize the facts of gemmation with the germ-layer theory, and attempts to explain budding in Ascidians as a process of re-generation, by which the organs of the parent or their germ-layersgive rise to the corresponding organs in the bud, have signally failed. Figs. 29 and 30 show the buds in the Botryllidac, after Pizon,who has followed day by day the changes of growth in young colonie9of Botryllidae, tracing the rise of successive generations of budsand the degeneration of their parents. The buds are parietal,arising from the walls of the peribr
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectencyclo, bookyear1910
