. Text-book of embryology. Embryology. IV ARCHINEPHRIC DUCT 241 2nd. above but making full allowance for this it seems impossible to escape the admission that in many forms (Petromyzon, Lung-fishes, most Amphibians, Teleosts and probably actinopterygian Ganoids) the duct is prolonged backwards by a process of this kind. It being accepted that in a number of Anamnia a large part of the archinephric duct arises in development by being split off from the mesoderm, we are faced by the problem how this mode of develop- ment is to be correlated with the mode of development by fusion of the outer end
. Text-book of embryology. Embryology. IV ARCHINEPHRIC DUCT 241 2nd. above but making full allowance for this it seems impossible to escape the admission that in many forms (Petromyzon, Lung-fishes, most Amphibians, Teleosts and probably actinopterygian Ganoids) the duct is prolonged backwards by a process of this kind. It being accepted that in a number of Anamnia a large part of the archinephric duct arises in development by being split off from the mesoderm, we are faced by the problem how this mode of develop- ment is to be correlated with the mode of development by fusion of the outer ends of tubule rudi- ments. It may be suggested that what has happened is that the development has been acceleratedâas often happens âby skipping over the early stages. The mode of develop- â >, my- ment in question may have been derived from the more primitive mode by the omission of the separate tubule stage and the passage at once to the stage in which the tubule ends are fused into, a continu- ous structure. In some cases however the primitive mode of development has undergone a further modi- fication. This is exemplified by PolypterUS (Graham Kerr, section through Polypterus 1907) where the hinder portion of stage 23 at level of cloacal opening. of the duct appears to be formed by bodily conversion of the series of nephrotomes. These are not segmented but form a continuous structure which becomes converted directly into the archinephric duct. In whichever way the archinephric duct completes its extension backwards, it eventually comes to open into the cloaca. This is, in the great majority of Vertebrates, described as coming about by fusion of the previously freely-ending tip of the duct with the cloacal wall. It is obvious that such a process cannot correspond with what happened during evolution as the duct must have had its posterior aperture throughout in order to perform its function. It is possible that a clue to the evolutionary origin of the c
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