MARSUPIALIA. 



325 



dent half a line below the origin of the pedicle. 

 (See the foetus in Jig. 141.) 



The small intestine above the pedicle was 

 disposed in five folds. The first from the 

 stomach or duodenum curved over the vitel- 

 line vein, and the remaining folds were dis- 

 posed around both the vitelline vessels. From 

 the ccecum, which was given off from the re- 

 turning portion of the umbilical loop of the 

 intestine, the large intestine passed backwards 

 to the spine, and was then bent, at a right angle, 

 to go straight down to the anus. The stomach did 

 not present any appearance of the sacculated 

 structure so remarkable in the adult, but had 

 the simple form of a carnivorous stomach. 

 The liver consisted of two equal and symme- 

 trically disposed lobes. The vena portae was 

 formed by the union of the vitelline with the 

 mesenteric, and doubtless the other usual veins, 

 which were, however, too small to be distinctly 

 perceived. The diaphragm was perfectly 

 formed. 



The vena cava inferior was joined, above 

 the diaphragm, by the left superior cava, just 

 at its termination in a large right auricle. The 

 ventricles of the heart were completely joined 

 together, and bore the same proportions to each 

 other as in the adult, a perfection of structure 

 which is not observed in the embryos of ordi- 

 nary Mammalia at a corresponding period of 

 development. The pulmonary artery and 

 aorta were of nearly the same proportionate 

 size as in the adult: the divisions of the pul- 

 monary artery to the lungs were at least double 

 the size of those observable in the embryo of a 

 sheep three inches in length. The ductus 

 arteriosus, on the contrary, was remarkably 

 small. The aorta, prior to forming the de- 

 scending trunk, dilated into a bulb, from which 

 the carotid and subclavian arteries were given 

 off. 



The lungs were of equal size with the heart, 

 being about a line in length, and nearly the 

 same in breadth : they were of a spongy tex- 

 ture and of a red colour, like the veins, from 

 the quantity of blood they contained. This 

 precocious development of the thoracic viscera 

 is an evident provision for the early or prema- 

 ture exercise of the lungs as respiratory organs 

 in this animal : and on account of the simple 

 condition of the alimentary canal, the chest 

 at this period exceeds the abdomen in size. 



The kidneys had the same form and situ- 

 ation as in the adult. The supra-renal glands 

 were half the size of the kidneys. 



The testes were situated below the kidneys, 

 and were one-half larger than those glands, the 

 superiority of size depending on their large 

 epididymis, with the adherent remains of the 

 Wolffian body. They continue within the ab- 

 domen for six weeks after uterine birth. 



At a later period of uterine development, 

 when the foetus, measured in a straight line 

 from the mouth to the root of the tail, is ten 

 lines in length, the urachus expands into a small 

 allantois Cd, jig. 141), of a flattened pyriform 

 ngure,and finely wrinkled external surface. This 

 bag insinuates itself between the amnios and 

 chorion, currying along with it two small hypo- 



gastric arteries and an umbilical vein, but not 

 establishing by their means an organized and 

 vascular surface of the chorion by which a 

 placental attachment is formed between the 

 ovum and the womb. The allantois depends 

 freely from the end of the umbilical chord, 

 and has no connexion at any part of its cir- 

 cumference with the adjoining membrane. Its 

 office is apparently that of a receptacle of 

 urine. 



The vitellicle or umbilical sac presented the 

 same large proportionate size and vascular 

 structure as in the first described foetus. The 

 chorion which enveloped this fetus and its 

 appended sacs was adapted to the cavity of the 

 uterus by being disposed in innumerable folds 

 and wrinkles. It did not adhere at any part 

 of its surface to the uterus, but presented a 

 modification not present in the chorion of the 

 earlier foetus, in being partially organized by the 

 extension of the omphalo-mesenteric vessels 

 upon it from the adherent vitellicle. The di- 

 gits of the hind legs were distinctly formed in 

 this embryo. 



The new-born foetus of the great Kangaroo 

 does not exceed, as we have already shown, 

 one inch in length ; its external characters have 

 been already described. Dr. Barton has given 

 the following account of the Opossum ( Didel- 

 phys Virginiana) at an analogous period. 

 " I have been so fortunate as to ascertain the 

 size and weight of several embryos imme- 

 diately after their exclusion from the uterus. 

 One of them weighed only one grain ! The 

 weight of each of the six other young ones 

 was but little more than this. The young 

 opossums, unformed and perfectly sightless as 

 they are at this period, jind their way to the 

 teats by the power of an invariable, a deter- 

 minate instinct" (qu.?). " In this new domi- 

 cilium they continue for about fifty days, that 

 is, until they attain the size of a common 

 mouse ( Mus musculus), when they begin to 

 leave the teats occasionally, but return to them, 

 again until they are nearly the size of rats. 



" At the end of about fifty or fifty-two days 

 from its first reception in the pouch the eyes of 

 the young begin to open. 



" I have found that the same embryon has 

 increased in weight 531 grains in sixty days, 

 that is, at a rate of almost 9 grains daily. The 

 animal attains to nearly its full growth in about 

 five months; but never, I believe, (in our lati- 

 tudes I mean,) procreates the first year of its 

 existence. 



" On the 21st of May, upon looking into 

 the box which contained the female Opossum, 

 I found that she had just excluded from her 

 uterus seven embryons; the smallest of which 

 scarcely weighed one grain, another barely two 

 grains, and the remaining five (taken together) 

 exactly seven grains.'' 4 



In the Kangaroo about ten months elapse 

 before the mammary foetus quits the pouch: 

 it has, prior to this period, quitted the nipple, 



* Barton, in Annals of Philosophy, vi. (1823), 

 p. 349. " Facts, Observations, and Conjectures relative 

 to the Generation of the Opossum." 



