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Show 1885.] MR. J. B. SUTTON ON HYPERTROPHY. 443 of the succeeding teeth (canines and premolars in rodents) became reduced to a minimum, so that diminished usage would also tend to lower the vascular supply: thus the two factors, diminished function and diversion of nutrient fluid, have brought about extreme atrophy and, in many cases, total disappearance of teeth. Numberless other instances might be quoted, indeed they must readily suggest themselves to any mind that carefully meditates on the matter. The cases of Mesoplodon and Monodon seem to me to be very extreme examples of this remarkable process. I propose now to consider some examples of hypertrophy as they affect the reproductive organs; and shall adduce evidence to show that this process is one of the probable causes of division into sexes. Many anatomists are of opinion that hermaphroditism is the primitive condition of the sexual organs. Hermaphrodites are found in every group of the animal kingdom, but, except in some of the lowest forms (such as Ctenophora and Chrysaora) among the Ccelenterata, self-fertilization appears to be wholly exceptional, and in those forms in which it occurs the entire process is of very simple character. The rule in hermaphrodites appears to be this :-The male organs in one animal are used to impregnate the female organs of another, or vice versa. Even in the Cestoda, where there are arrangements favourable to self-impregnation, we have no positive evidence that it takes place. From this arrangement it would easily come to pass that if one animal used the male portions of its reproductive organs more freely than the female parts, they would, as a result of increased function, hypertrophy. In the first portion of this paper I emphasized the fact that any marked degree of hypertrophy in one organ nearly always leads to dwarfing of the correlated organ or sets of organs; hence in the example considered, the female portions of the hermaphrodite organs remain dwarfed or in statu quo. This peculiarity would in the natural course of events be transmitted to the offspring, until at last the differentiation attains such a high degree, that unless hypertrophy of one set of organs occurs in each individual, propagation is impeded. Evidence on this point is afforded by the developmental history (ontogeny) of any mammal. Whilst the two sets of reproductive organs, male and female, up to a certain point maintain the same degree of growth, it is impossible to determine the sex of the embryo. As soon as one set begin to enlarge at a greater rate than the other, the sex becomes pronounced. The remaining organs may eventually disappear or merely exist in such a rudimentary condition as to be discerned only by the most diligent search. Indeed this process has been observed to occur in a very complete manner in a single group, the Myzostomida. Dr. L. von Graff, in his " Report on the Myzostomida " collected during the voyage of the ' Challenger,' has noted the following'interesting facts. Some species (Myzostoma tenuispinum, M. ivillemoesii, M. inflator, and murrayi) are originally descended from androgynous forms in which the organs of one sex have become gradually aborted; for in Myzo- |