. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 117 weight must be attached to this testimony. Personally I think that the collective evidence which I now propose to state (under five headings) amounts to proof. (A) Though the facts of ontogeny rarely afford conclusive evidence as to the past history of any given structure, owing to the modifica- tions in the palingenetic series of changes which may be induced by the conditions of development in the organism, yet the very fact that ontogeny does not in any way countenance a given theory is of value: moreover the facts of o
. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 117 weight must be attached to this testimony. Personally I think that the collective evidence which I now propose to state (under five headings) amounts to proof. (A) Though the facts of ontogeny rarely afford conclusive evidence as to the past history of any given structure, owing to the modifica- tions in the palingenetic series of changes which may be induced by the conditions of development in the organism, yet the very fact that ontogeny does not in any way countenance a given theory is of value: moreover the facts of ontogeny may be useful as affording confirma- tion of conclusions arrived at from other evidence and it is solely for this reason that I will describe briefly the development of the piston musculature and piston. The piston musculature is developed on the mid-ventral side of the gut just in front of the heart and shows no dorsal or lateral exten- sions. As the pericardial region, with the gills, shifts backwards owing to the elongation of the neck region, the muscles also extend backwards as the result of the modi- fication of embryonic tissue situa- ted in the mid-ventral line (Bujor, (5), Dean, (7) and in the adult stretch from the region of the mouthcavity to the region of the anterior gill pouch (Myxinoids) or to the pericardium (Petromyzontes). This course of development appears to me to be quite inconsistent with the view that the piston musculature was primitively attached to the skull (which must have degenerated considerably if it ever supported masticatory muscles!) and worked a mandible. It is indeed inconceivable that muscles underlying the gut and extending right back, in Petromyzontes, to the pericardium, should ever have been associated with a jaw. Also we may well wonder how the animal fared during the transitional period in which the man- dible was becoming converted into a dentigerous piston and its muscles, after relinquishing their connection with the skull a
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