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The following account of the arterial system of the ganoid Polyodon was begun in the Anatomical Department of Washington University and completed in the Department of Comparative Anatomy a t Harvard Medica School. The writer gratefully acknowledges his indebtedness t o the staffs in both places. The work is approached entirely from a morphological viewpoint and only the gross anatomical relationships are considered. The material used has been ch:efly fish of about a meter in length which were secured in the vicinity of St. Louis. These were injected w;th gelatin and starch masses, the latter proving quite satisfactory for present purposes. The results have been checked by a study of. serial sections of a 74 mm. specimen which has already been briefly described by the writer (Danforth, '11).Allis's ('11) paper on the pseudobranchial and carotid arteries of Polyodon did not appear until t,he present work had been finished except for the drawings. Since that paper only partially covers the field attempted here and moreover some comparison of our results seems desirable the paragraphs on the pseudobranchial and carotid vessels are retained without very much condensation. PERICARDIUM AND HEARTThe pericardium has the usual conical or rounded form with the base directed caudally against the septum transversum. A dorsal and two lateral faces are vaguely indicated. The lining is a uniform serous membrane with no macroscopic openings except that of the pericardio-peritoneal canal. This is a rather 409

Phases of morphology which relate to the ultimate nature and meaning of homology have remained consistently baffling. For three quarters of a century it lias been considered as axiomatic that each animal species has had a long phylogenetic histor:-during which it lias progressively deviated from earlier ancestral forms; and, for a somewhat lesser period, it has been considered as almost equally axiomatic that manifest evolutionary change is merely the outward expression of ultra-microscopic changes in the germplasm.But even if both these postulates are accepted in full, there still remains a formidable array of subsidiary questions which are as yet unanswered. Some of the latter have been discussed, from rather divergent points of view, by Bateson (1894), Boyden ( '43), Hubbs ( '44), Danforth ( '25), and others. One of the most difficult of them is that of assessing the degree of phylogenetic and ontogenetic autonomy possessed by individual structural elements such, for example, as a metacarpal bone, or the tibialis anterior muscle.Polydactylv in the cat provides excellent material for the study of some of these morphological problems. The condition is one of 110 great rarity and parallels, at least superficially, similar manifestations in man and several other species. Descriptions of feet from individual polydactyl cats have been published bJ-several authors, notably Wilder (1868), Howe

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