may become largest in the adult. At birth, the nostrils are large, with a high rim; but the eyes are covered beneath the skin, and the ears are represented by small elevations on the sides of the head, while the lips have a remarkable development and peculiar covering, which reminds us of the first embryonic traces of the duck-like bill of ornithorhynchus. The tongue has a peculiar papillated groove above, to fit the nipple, and three very large papillæ on its base. The larynx and epiglottis project so high into the broad pharynx that the milk swallowed passes in two currents, one on either side. A very large three-lobed thymus gland lies above the heart. Only a rudiment of this exists in the adult. The heart is large, and situated on the median line. Its position changes somewhat as it grows older. The lungs are equal in size. Curiously, the œsophagus enters the stomach near its pyloric end. A very large gland lies on the cardiac end of the stomach. Prof. Owen, speaking of the character of the stomach in marsupials, says: "The stomach is simple in the genera Didelphys, Myrmecobius, and Parameles, and likewise simple in Dasyurus and Phalangista; also in the kaola and wombat, but in these two animals it is provided with a glandular apparatus situated to the left of the cardiac orifice." This is so large in the young Didelphys, that it is curious it does not exist when the animal is fully developed. In the possession of this organ, the young opossum agrees with the old kaola and wombat, but the old opossum has developed a stage further, so that the organ becomes rudimentary, or disappears. The cæcum is relatively twice as large as in the adult. The optic lobes of the brain were relatively larger, and the cerebral lobes somewhat smaller than when full grown. When first born, the male and female are, externally, exactly alike; clitoris and penis are large external organs, just in front of the vent, and so much alike, that it is impossible to distinguish the female from the male by these parts, so markedly different at maturity. Even in the oldest specimens studied, the same similarity of size and form of these parts exists, but the female organ stands nearer to the margin of the vent. Some time after birth, the testes descend into a large scrotum, which has a peculiar position, being at some distance in front of the penis. This is the first external sexual difference, for, although the marsupium begins to appear about the same time, it is remarkable that the male at first has as good a pouch as the female. This is first seen as a cluster of very low papillæ on the abdomen, nearly surrounded by a slight ridge. Slowly this ridge rises higher, and the depression extends itself deeper and more laterally, while the outer edge becomes a fold of skin growing inward toward the median line, until, finally, only a narrow opening is left. The marsupium of the male never becomes fully developed, but gradually diminishes in size; still it was well marked in the largest specimens studied.
To the embryologist every one of these curious facts has great