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Show 338 MR. W, WOODLAND ON THE [Apr. 21, connection with respiration cannot have constituted its primitive raison d'etre. Thus, observing the necessity for the development of the diaphragm, we find that its variations in disposition and contour found in the Struthiones and Mammalian orders fully confirm the conclusion deduced 011 a priori grounds. Another organ which, though not peculiar to the Mammalia, is yet a diagnostic feature of terrestrial vertebrates, is the metanephric kidney, and that there is possibly a relationship between terrestrial modes of locomotion and the evolution of this organ I will now endeavour to show. It is well established that the metanephros is a development . of the definitive mesonephros which has lost its nephrostomata, acquired a separate duct, and become more or less concentrated in form. It has been pointed out above that concentration of structure is an essential concomitant of that increase of impulsiveness which results from the increased activity of the animal under conditions which involve marked reactions between the body and the medium or substratum. That this is so in the case of the kidney, it is only necessary to compare the elongated mesonephric bodies of Pisces and aquatic Urodeles with the same organs of the terrestrial Amphibia and the metanephric bodies of Reptilia, and again to compare these latter with their representatives in Mammalia, in which both concentration of structure and impulsiveness of locomotion attain their maxima. With regard to the loss of nephrostomata, the same relation holds. In all Pisces, with the exception of certain Elasmobranchii, the mesonephric bodies retain connection with the coelom by means of the nephrostomial tubules. Whether the absence of these fragile structures in Elasmobranchs (their function with regard to the coelom perhaps being assumed by the abdominal pores) is to be attributed to the fact that these fishes either are or are not descended from the most active members of their class (and it must be remembered that they are surface forms) is not certain, but it is possible. In the aquatic Urodeles the nephrostomes are present, but in the terrestrial Anura they have completely lost then connection with the coelom (which latter, as in Elasmobranchs, has had to discover another means of exit for its waste products). The causal relation between disruption of the nephrostomata and the adoption of a terrestrial life is here clearly shown by the ontogeny. In the development of the frog, the mesonephros is at first in communication with the coelom by means of the nephrostomata, but at the period of metamorphosis this connection is severed, and the kidney, becoming more concentrated, finally assumes the definite form of the adult structure. Needless to say, the metanephros of Reptilia, Aves, and Mammalia is totally devoid of nephrostomes, that of the last not possessing them at any stage of development. The division of the mesonephros into two portions, one coming into relation with the testes, and the other-the definitive mesonephros-retaining its |
