326 



MARSUPIALIA. 



and occasionally protrudes its head and changes 

 its position in the pouch. 



The anatomical condition and progressive 

 development of the mammary foetus of the 

 Marsupialia offer a subject of highly interest- 

 ing research, especially if compared with the 

 same circumstances in the uterine fetus of an 

 equal sized and analogous placental species. 

 Much still remains to be done in this chapter 

 of the history of Marsupial generation ; at 

 present J have to offer the following obser- 

 vations. 



By comparing the new-born Kangaroo with 

 a similarly sized fetus of a sheep, we find that, 

 although, in the Kangaroo, the ordinary laws 

 of development have been adhered to in the 

 more advanced condition of the anterior part 

 of the body and corresponding extremities, yet 

 that the brain does not present so dispropor- 

 tionate a size ; and the same difference is ob- 

 servable in the uterine fetus of the Kangaroo, 

 even when compared with the same sized em- 

 bryo of an animal of an inferior class, as the 

 bird. This difference, I apprehend, is owing 

 to the rapidity with which the heart and lungs 

 acquire their adult structure in the Kangaroo, 

 whereby the passage of the purer and more 

 nutritious blood through the foramen ovale and 

 left auricle to the primary branches of the 

 aorta and so to the brain is impeded. The 

 brain, however, of the mammary fetus, though 

 exhibiting a low degree of development, yet is 

 of a firmer texture than in a similarly sized 

 foetus of a sheep, and attains its ultimate pro- 

 portion by a more gradual process of growth. 



In a mammary fetus, one inch and a half 

 in length, the urinary bladder is largely deve- 

 loped, and adheres by its apex to the perito- 

 neum, exactly opposite that part of the abdo- 

 minal integument where a small linear ridge 

 indicated the previous attachment to the umbi- 

 lical chord and appendage. There are also 

 minute but distinct traces of umbilical arteries 

 running up the sides of the bladder to this point 

 of attachment. As the urinary bladder be- 

 comes afterwards expanded in the abdomen, 

 the peritoneum is gradually, as it were, drawn 

 from this part of the abdominal parietes, form- 

 ing an anterior ligament of the bladder. In a 

 mammary fetus of the Kangaroo about a month 

 older than the above, there was at the superior 

 part of this duplicature a small projecting point 

 from the bladder, like the remains of a ura- 

 chus ; but the fundus, now developed con- 

 siderably above this point, was covered with 

 a perfectly smooth layer of peritoneum ; and 

 it is this modification, I apprehend, which 

 led Hunter to suppose that there was no trace 

 of urachus or umbilical arteries in the fetuses 

 of the Marsupialia. In the Sloth, the Manis, 

 and the Armadillo, the urachus is continued 

 in the same manner from the middle of the 

 anterior part of the bladder, and not from the 

 fundus. 



In neither of the above fetuses of the Kan- 

 garoo was there any corresponding trace of 

 umbilical vein, although there was a distinct 

 ligamentum suspensorium hepatis, formed by 

 a duplicating uf ihe peritoneum descending 



from the diaphragm to the notch lodging the 

 gall-bladder, and not entering, as usual, the 

 fissure to the left of that notch : the allantois is 

 too small, and its function too limited for the 

 preservation of any permanent trace of its 

 peculiar vein. 



The small intestines in the mammary fetus, 

 one inch and a half long, when compared 

 with those of the uterine fetus above de- 

 scribed, were found to have acquired several 

 additional convolutions ; the fold to which the 

 umbilical vesicle had been attached was still 

 distinct, but now drawn in to the back of the 

 abdomen. The caecum was much elongated, 

 but the colon proportionately not more deve- 

 loped than in the uterine fetus ; the subse- 

 quent modification, therefore, of the large in- 

 testines seems evidently destined to complete 

 the digestion of the vegetable food. 



The stomach was not sacculated, but the 

 division between the cardiac and middle com- 

 partments was more marked than in the uterine 

 fetus. The liver had now advanced in its 

 development beyond the oviparous form which 

 it presented in the uterine fetus, the right lobe 

 being subdivided into three. The supra-renal 

 glands bore the same proportionate size to the 

 kidneys. The testes were still larger than the 

 kidneys, and were situated below them, not 

 having yet passed out of the abdomen : this 

 takes place when the mammary fetus is about 

 three inches long from the nose to the root of 

 the tail. The ductus arteriosus was distinct in 

 the small mammary fetus, but I could not 

 perceive any trace of the thymus gland. Is 

 this gland unnecessary on account of the pre- 

 cocious development of the lungs ? or because 

 of the small size and gradual growth of the 

 brain ? The latter appears the more probable 

 condition of its absence, as in the ovovivipa- 

 rous classes with small and simple brains 

 the thymus gland is rudimental or of doubtful 

 existence. 



Notwithstanding that the new-born Kanga- 

 roo possesses greater powers of action than the 

 same sized embryo of a sheep, and approxi- 

 mates more nearly in this respect to the new- 

 born young of the rat, yet it is evidently in- 

 ferior to the latter. For, although it is enabled 

 by the muscular power of its lips to grasp and 

 adhere firmly to the nipple, it seems to be 

 unable to draw sustenance therefrom by its 

 own unaided efforts. The mother, as Professor 

 Geoffrey and Mr. Morgan have shown, is 

 therefore provided with the peculiar adaptation 

 of a muscle (analogous to the cremaster) to the 

 mammary gland, for the evident purpose of 

 injecting the milk from the nipple into the 

 mouth of the adherent fetus. Now it can 

 scarcely be supposed that the fetal efforts of 

 suction should always be coincident with the 

 maternal act of injection ; and if at any time 

 this should not be the case, a fatal accident 

 might happen from the milk being forcibly 

 injected into the larynx, unless that aperture 

 were guarded by some special contrivance. 

 Professor Geoffrey first described the modi- 

 fication by which this purpose is effected ; and 

 Mr. Hunter appears to have anticipated the ne- 



