OCR Text |
Show 654 PROF. HUXLEY ON THE [Dec 14, 3. The openings of the ureters are entocystic; that is to say, the ureters open into what is called the " base" of the bladder, in front of the narrowed "neck" by which it passes into the tubular "urethra." This means, I conceive, that, morphologically, the bladder of the Marsupial represents the bladder of the Monotreme + the anterior part of the genito-urinary passage-the so-called " trigonum," if not more, of the bladder of the Marsupial, being the homologue of that anterior segment of the genito-urinary passage ot the Monotreme. 4. There is a distinct and long vagina, quite separated from the cystic urethra, in the female; and the oviducts are differentiated into uterine and Fallopian portions. 5. The penis is large, and the corpora cavernosa are connected by fibrous tissue and muscles with the pelvis. The spongy body has a large bifurcated bulb ; and Cowper's glands are very largely developed. 6. The vertebrae have distinct epiphyses. 7. The malleus is small; and its connexions are similar to those which it possesses in the higher mammals. The incus is relatively larger, and the stapes more or less stirrup-shaped. 8. The coracoid is short, does not articulate with the sternum, and becomes ankylosed with the scapula. 9. The hip-girdle is provided with epipubes, usually of large size and well ossified ; and the iliac axis is inclined at a small angle to the sacral axis. 10. The corpus callosum is small. 11. In the few forms of which the foetus is knowm there is no allantoic placenta; while the umbilical sac is so large that the possibility of the existence of a transitory umbilical placentation must be taken into account. It will be observed that in the characters I, 2, 3, 4, 5, 6, 7, 8, and the latter part of the 9th, the Marsupials agree with the higher mammals; while in the former part of the 9th, the 10th, and the 11th they present Prototherian characters. So far, therefore, they constitute an intermediate type between that of the Prototheria and that of the higher mammals, which may be termed that of the Metatheria. And if there were any known animals which combined these characters, with a complete double dentition, unmodified pentadactyle manus and pes, and normal uterogestation, they would furnish us with the exact transition between the Prototheria and the higher mammals, which must have existed if the law of evolution is trustworthy. No known Marsupial, however, possesses these additional characters. None has more than a single successional tooth on each side of each jaw ; and, as Prof. Flower (to whom we owe the highly important demonstration of this fact) has pointed out, the question arises whether we have here a primary dentition with only one secondary tooth, or a secondary dentition with only one tooth of the primary set left. I have no doubt that the answer given to this question by Prof. Flower is correct, and that it is the milk- |