COMPARATIVE STYDIES OF THE SPLEEN IN SUBMAMMALIAN VERTEBRATES : Ⅰ. TOPOGRAPHICAL ANATOMY AND RELATIVE WEIGHT OF THE SPLEEN

A comparative survey was made of the spleen of submammalian vertebrates, from Cyclostomata to Aves, with respect to the tepographical anatomy and relative weight of the organ. There are considerable variations in the topography of the spleen in various submammalian vertebrates. However, an elongated spleen extending the entire length of the gus is regarded as the ancestral from, and the variations in the shape and position od this organ among different groups are explained on the basis of an assumption that the entire dorsal mesentery retains phylogenetically a developmental potentiality of forming splenic tissue, but an arrest of development either at the caudal or at the cranial protion of the mesentery usually takes place. The relative weight of the spleen, expressed as percentage of body weight, is also wuite variable. An exceedingly high value was obtained for the spleen of Mustelus manazo ( a kind of shark ) as great as 2.966±0.1855%, but the corresponding valuse were much power in other submammalian vertebrates, varying within a wide renge between 0.049% and 0.495%. The presence of a relatively heavy spleen was noticed in some species of Pisces, Urodela and Aves.