MARSUPIALIA. 



327 



ressity for such a structure, for he dissected 

 ?wo small mammary foetuses of the Kangaroo 

 for the especial purpose of showing the rela- 

 tion of the larynx to the posterior nares. The 

 epiglottis and arytenoid cartilages are elongated 

 and approximated, and the rima glottidis is 

 thus situated at the apex of a cone-shaped 

 larynx, (Jig. 142, c ,) which projects, as in the 

 Cetacea, into the posterior nares, where it is 

 closely embraced by the muscles of the soft 

 palate. The air-passage (6) is thus completely 

 separated from the fauces, and the injected 

 milk passes in a divided stream on either side 

 the larynx to the oesophagus. 



Fig. 142. 



Nipple, and head of Mammary Foetus, Kangaroo. 



Thus aided and protected by modifications 

 of structure, both in the system of the mother 

 and its own, designed with especial reference 

 to each other's peculiar condition, and afford- 

 ing, therefore, the most irrefragable evidence 

 of creative foresight, the small offspring of the 

 Kangaroo continues to increase, from suste- 

 nance exclusively derived from the mother, for 

 a period of about eight months. During this 

 period the hind legs and tail assume a great 

 part of their adult proportions; the muzzle 

 elongates ; the external ears and eyelids are 

 completed ; the hair begins to be developed at 

 about the sixth month. At the eighth month 

 the young Kangaroo may be seen frequently 

 to protrude its head from the mouth of the 

 pouch, and to crop the grass at the same time 

 that the mother is browsing. Having thus 

 acquired additional strength, it quits the pouch 

 and hops at first with a feeble and vacillating 

 gait, but continues to return to the pouch for 

 occasional shelter and supplies of food till it has 

 attained the weight of ten pounds. After this 

 it will occasionally insert its head for the pur- 

 pose of sucking, notwithstanding another foetus 

 may have been deposited in the pouch ; for the 

 latter, as we have seen, attaches itself to a dif- 

 ferent nipple from the one which had been pre- 

 viously in use. 



Mammary Organs. In the young Marsu- 

 pial, as Mr. Morgan was the first to observe 

 in the Kangaroo, the nipples are not visible, 

 but are indicated by the orifices of a kind 

 of cutaneous preputial sheath in which they 

 are concealed. M. Laurent has noticed a 

 similar condition of the nipples in a mam- 

 mary foetus of an Opossum and a Perameles. 

 I have also observed it in the mammary fostus 



of a Petaurist and Dasyure : it is doubtless, 

 therefore, common to all Marsupials. 



Once naturally protruded and the preputial 

 sheath everted, the nipples, in the Kangaroo 

 at least, continue external. They are longer 

 and more slender than in other quadrupeds, 

 and when in use generally present a terminal 

 expansion (jig. 142, d). This part lies in 

 a deep longitudinal fossa on the dorsum of 

 the tongue (a, fig. 142); and the originally 

 wide mouth of the uterine fetus is changed 

 to a long tubular cavity, with a terminal sub- 

 circular or triangular aperture just large enough 

 to admit the nipple, to which the young Mar- 

 supial thus very firmly adheres. 



In the Phascogale, in which the nipples are 

 relatively larger than usual, and of a subcom- 

 pressed clavate form, the young, when grown 

 too large to be carried in the pouch, are 

 dragged along by the mother, if she be pursued, 

 hanging by the nipples. 



The number of nipples bears relation in 

 the marsupial, as in the placental Mammalia, 

 to that of the young brought forth at a birth; 

 although from the circumstance of the produce 

 of two gestations being for a short time suckled 

 simultaneously, the nipples are never so few. 

 Thus the uniparous Kangaroo has four nipples ; 

 of which the two anterior are generally those 

 in use : the Petaurists, which bring forth two 

 young at a birth, have also four nipples : the 

 Thalycine has four nipples : the multiparous 

 Virginian Opossum has thirteen nipples, six on 

 each side and the thirteenth in the middle. In 

 the Didelphys Opossum there are nine nipples, 

 four on each side and one in the middle. The 

 Didelphys dorsigera has the same number of 

 nipples, although six is the usual number of 

 young at a birth (fig. 143). In the Phas- 

 cogale penicillata there are eight nipples ar- 

 ranged in a circle. The Perameles nasuta has 

 the same number of nipples arranged in two 

 slightly curved longitudinal rows; this Mar- 

 supial has three or four young at a birth. 



The nipple in all the Marsupials is i m per- 

 forate at the centre; the milk exudes from six 

 to ten minute orifices arranged round the apex. 

 It increases in size with the growth of the 

 mammary foetus appended to it. 



The mammary gland has the same essential 

 structure as in the ordinary Mammalia; it has 

 no cavity or udder; its chief peculiarity arises 

 from its being embraced by the muscle, already 

 noticed, which has the same origin and course 

 as the cremaster muscle in the male. 



Marsupial pmich. The development of the 

 pouch is in an inverse ratio to that of the uteri 

 and directly as that of the complicated vaginse : 

 thus it is rudimental in the Dorsigerous Opos- 

 sum, which has the longest uteri and the sim- 

 plest vaginas : we may conclude therefore that 

 the young undergo a greater amount of deve- 

 lopment in the womb in this and allied 

 species.* 



* Is there any csscnlial modification of the mem- 

 branes of theovum in these small Marsupials? The 

 means of determining this question arc most de- 

 sirable. 



