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Show 1 9 0 3 . ] TRANSPOSITION OF MAMMALIAN TESTES. 3 3 9 urinary function, is evidently a physiological phenomenon, though the ultimate structural separation of the two parts must largely be attributed to the influence of greater activity of the organism. Also the secondary development of the definitive mesonephros is indirectly due to increased activity, since the work of the excretion resulting from the latter is largely increased. In fact, the whole ontogeny of the kidney-the successive developments of the pro-, meso-, and metanephros in space and time, the differences obtaining between the pro- and mesonephric tubules, the disappearance of the pronephros, &c., &c.-can be directly or indirectly attributed to the increasing activity of the organism in the course of phylo-geny, as a little thought will show. With regard to the position of the kidney-a body of considerable mass, and in Amniota similarly disposed to the testes-no relation between such and the impulsiveness of the animal being traceable, it is evident that some other factor must be involved, and enquiry shows that the localisation of the kidney is correlated with the total activity of the animal. Excretion being facilitated by proximity to a vigorous blood-supply, we find that in active animals the kidney tends to be situated anteriorly, i. e. nearer the heart; on the other hand, close connection with the vent to the exterior is similarly advantageous, and in inactive animals, such as Reptilia and TJrodeles, we find the kidneys situated at the posterior extremity of the coelom. The kidneys of mammals are retained in position by a circumambient development of areolar tissue, which usually contains much fa t; support is also contributed by the " tonicity " of the muscles of the abdominal wall and by the terminal ribs. In conclusion, I must acknowledge my indebtedness for details respecting the habits and anatomy of the mammalian orders to the following authors and works: Flower and Lydekker, Owen, Wiedersheim, Beddard and Gegenbaur, Lydekker's 1 Royal Natural History,' ‘ Mammals' by Vogt & Specht, Schmeil's ‘ Textbook of Zoology,' and Allen's & Lloyd's editions of Jardine's ‘ Naturalist's Library.' I also wish to thank Prof. Minchin, Mr. J. T. Cunningham, and Mr. H. S. Shelton for kindly reading through the manuscript and for making several suggestions and criticisms. A p p e n d ix . In respect to the non-descent of the ovary in the Mammalia, it is as well to emphasise one factor ensuring its retention. Assuming that the females of mammals are approximately as active as the males, it is evident that if the mammalian ovum had retained its yolk (i. e. if the mammalian ovary possessed the size and mass of that of the Sauropsida and Monotremes), the retention of the ovary within the body-cavity by the development of special ligaments, &c., would have been, if not impossible, yet extremely hazardous. And such being the case, it would obviously benefit the race if the potency of one of these two factors were decreased. |