OPOSSUMS AND THEIR YOUNG. i 



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may become largest in the adult. At birth, tbe nostrils are large, 

 with a high rim ; but the eyes are covered beneath the skin, and 

 the ears are rej^resented 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 tliree very large papillae 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 some- 

 what as it grows older. The lungs are equal in size. Curiously, the 

 cesophagus 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 glandu- 

 lar 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 csecum 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 diflerent 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 

 scrotnm, which has a peculiar position, being at some distance in 

 front of the penis. This is the first external sexual diflerence, 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 papillae on the abdomen, 

 nearly surrounded by a slight ridge. Slowly this ridge rises higher, 

 and the depression extends itself deeper and more laterally, while tlie 

 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 



