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Show 1 9 0 3 . ] TRANSPOSITION OF MAMMALIAN TESTES. 3 2 5 beyond the surface of the body-wall, besides being a necessary result of more massive organs and the incidence of more powerful forces, is in itself a further means of preservation. In physical character, the testes, as already mentioned, are definite bodies of concentrated form, suspended dorsally in the body-cavity by the thin mesorchial membrane. Also investigation shows that, with the exception of the heart, the testes are the densest organs in the vertebrate body, and hence, as they possess appreciable volume, their mass is also considerable. Moreover, the testes (and ovaries) are distinctly separated from the remainder of the body, this separation (lending additional facility to transposition) arising out of the fact that the gonads are from their very nature individualised, reproduction essentially consisting of the separating off of a portion of the organism; and hence the body primarily serves as a mere carrier of these organs, which, unlike the kidneys e. g., bear no relation to the economy of the animal. From this it follows that the transposition of the testes is of no concern to the rest of the organism, since the process can cause no derangement of function in other portions of the body; and hence the testes, differing from all other organs in this respect, do not, under ordinary conditions, require to be retained in their primitive position by the special development of fixative structures. Thus, as regards their definiteness and concentration of form, their means of suspension, their superior density, and their structural and physiological separateness from the rest of the organism, the testes fully conform to the above-specified conditions favourable to transposition. The transposition of the testes occurring under conditions which permit the descent of these organs alone (and the testes alone have descended), and a cause capable of effecting this transposition solely existing in the Mammalia (in which group descent has alone occurred), it is probable that the latter phenomenon is the cause of the former. I hold that, in the majority of the Mammalia, the testis attachment has throughout the history of the race been constantly subjected to severe strains consequent on the character and conditions of mammalian locomotion, and that on account of the resulting disruption or distension of the mesorchium, the testis has migrated in a postero-ventral line (i. e. in an opposite direction to the forward and upward accelerations imparted), coming to lie at the terminal periphery of the body-wall and forming the scrotal protrusion. Just as when a man runs, a weight in his coat-pocket will periodically " drag " and ultimately wear a hole in the lining by constant distension, so the testis of mammals has responded to like forces resulting in " descent." I now proceed to consider the genital organs of the Mammalia as a whole, the general conformation of which amply confirms the foregoing conclusion, tending to show that the testes have, in every case, reacted in a degree proportional to the forces concerned. Indeed, the general correspondence between situation of the testes and grade of impulsiveness displayed in the various |
