Abstract
A comparative survey wa s made of the spleen of submammalian vertebrates, from Cyclostomata to Ayes, with respect to the topographical anatomy and relative weight of the organ.
There are considerable variations in the topo g raphy of the spleen in various submammalian vertebrates. However, an elongated spleen extending the entire length of the gut is regarded as the ancestral form, and the variations in the shape and position of 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 portion of the mesentery usually takes place.
The relative weight of the spleen, expressed as percentage of body weight, is also quite 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 values were much lower in other submammalian vertebrates, varying within a wide range between 0.049% and 0.495%. The presence of a relatively heavy sple en was noticed in some species of Pisces, Urodela and Ayes.
