768 ANATOMY OF VERTEBRATES. 



wider, and has the form of a transverse slit ; it is not circular. 

 Upon the upper lip, in the midline between the two nostrils, 

 is a small protuberance, corresponding to that in the young of 

 the Ornithorhynchus paradoxus, and wanting the cuticle. The 

 tongue is broad and flat, extending to the ( rictus oris,' but 

 very short in proportion to that of the parent, and of a very 

 different shape. The tail is much shorter than in the young 

 Kangaroo, and shows as much proportional size as in the full- 

 grown Echidna, in which it is a mere stump concealed by the 

 quills and hair. The head is proportionally longer and more 

 slender in the marsupial foetus of the Echidna than in that of the 

 Kangaroo, and already, at this early period, foreshows the cha- 

 racteristic elongation and attenuation of that part in the mature 

 animal. The form of the mouth, as a transverse slit, is a good 

 monotrematous character of the young at that period, since in all 

 true or teated marsupials the mouth of the mammary foetus has 

 a peculiar circular and tubular shape. A scarcely visible linear 

 cicatrix at the middle of the lower part of the abdomen is the 

 sole trace of umbilicus. A bifid, obtuse rudiment of penis or 

 clitoris projects from the fore part of the single urogenital or 

 cloacal aperture, and in advance of the base of the tail-stump. 

 Of the brain, the largest part is the mesencephalon, chiefly con- 

 sisting of a vesicular condition of the optic lobes. 



The fore limbs, in their shortness and breadth, foreshow the 

 characteristics of those of the parent, which may be said, indeed, 

 to retain in this respect the embryonic character with super- 

 induced breadth and strength. The digits have already some- 

 thing of the adult proportions, the first or innermost of the five 

 being the shortest, the others of nearly equal length, but gra- 

 duating shorter from the third to the fifth ; each digit is termi- 

 nated by a claw : in the hind limb, the second is already the 

 strongest and longest, the rest more rapidly shortening to the fifth 

 than in the fore leg ; the innermost, agreeably with the law of 

 closer retention of type in the embryo, though the shortest of the 

 five, is less disproportionally so than in the adult. The young 

 nestles its head and fore-limbs within the marsupial fossa, cling- 

 ing by its precocious fore claws to the skin or hairs of that part, 

 and imbibing by its broad, slit-shaped mouth the nutritious se- 

 cretion as it is pressed by the muscles acting upon the gland 

 from the areolar outlets of the ducts. 



§ 414. In Marsupialia. — In Marsupials the mammary glands 

 have a more compact form and minutely conglomerate structure 

 than in Monotremes. They are developed on each side the linea 



