320 
tion, &e. This theory, however, was aban- 
doned, or at least considerably modified after 
the publication of Dr. Barton’s letters relative 
to the breeding of the Opossum. The product 
of marsupial generation is then stated by M. 
Geoffroy to quit the uterus in the condition ofa 
gelatinous ovulum, comparable toa Medusa, and 
to become organically connected with the nipple 
by cme of vessels. He sup , there- 
fore, that a flow of blood followed the detach- 
ment of the mammary foetus from the nipple,* 
and even speculates upon the signification of 
the thyroid gland, on the strength of this hypo- 
thetical confluence of the maternal and fetal 
vascular systems in the marsupial tribe. + 
In Sinother essay M. Geoffroy abandons this 
inion, it having been proved by repeated 
° tion that the adhesion of the foetus to 
the nipple is by simple contact merely ;{ and 
finally, he falls into the opposite extreme, and 
from some appearances of an urachus which 
were pointed out to him in a very small foetus 
of an American Opossum, he describes them 
as vestiges of a placental organization. 
Mr. Brocyui:t in his elaborate and excellent 
Memoirs on the Structure and Development of 
the Mammary Organs of the Kangaroo, bears 
testimony to the simply mechanical mode of 
attachment between the mammary fetus and 
the nipple in the Kangaroo, and was the first 
to show that the young animal, in its blind 
and naked condition, prior to the act of volun- 
tary detachment,—the birth ‘ a la manitre des 
Marsupiaux,’ as it is called by M. Geoffroy,— 
would bear a separation from the nipple for 
two hours, and voluntarily regain its hold. The 
mammary fetus of the Kangaroo on which Mr. 
Morgan experimented was nearly the size of a 
fully grown Norway rat. Mr. Collie, surgeon 
R.N. in a letter addressed from New South 
Wales, and published in the Zoological Journal, 
(No. xviir. p. 239,) obtained the same result 
in detaching from the nipple of a smaller 
species of Kangaroo (Macropus Brunii) a 
mammary foetus, not two inches in length: he 
says, “I gently pressed with the tip of my 
finger the head of the little one away fist the 
teat of which it had hold, and continuing press- 
ing a little more strongly for the space of a 
minute altogether, when the teat which had 
been stretched to more than an inch came out 
of the young one’s mouth, and showed a small 
circular enlargement at its tip, well adapting it 
for being retained by the mouth of the sucker.” 
- Gockvey St. Hilaire, in detailing some observa- 
tions on a Kangaroo in the ‘ Annales des Sciences,’ 
observes, “ car le sang apergu 4 la litiére est un 
indice qu’a ce moment le foetus s’est detaché de la 
tetine et qu’il est né définitivement ala maniére des 
marsupiaux.” Vol. ix. p.342, 1827. 
t “* Des vaisseaux nourriciers se repandroient-ils 
des parties du pharynx le long et entre les lames de 
la trachée artére pour entrer fe ceeur, et (conjectare 
de M. Serres) le gland thyroide seroit-il le point 
de lear reunion?” Geoff. St. Hilaire, Mémoires du 
Muséum, tom. xix. p. 406, 1822. 
¢ Art. Marsupiaux, Dict. des Sciences Nat. tom. 
xix. 1823. 
§ Transactions of the Linnwan Society, vol. xvi. 
pp. 61, 455, 
MARSUPIALIA. 
—* An hour afterwards the young was ob- 
served still unattached, but in about two hours 
it had hold of the teat, and was actively em- 
pit sucking.” : — 
r. Barton’s very interesting observations on 
the American Opossum are as follows : the: 
female Didelphis Woapink sometimes produces — 
sixteen young ones at a birth. I have actually 
seen this number attached to the teats, but 
never a greater number. When they are first 
excluded from the uterus, they are not only 
very small but very obscurely shaped. The 
place of the future eyes is merely marked b: 
two pale bluish specks; we see no ears; it 
short the animal is a mere mis-shaped embryon. 
Its mouth, which is afterwards to become ve 
large, is at first a minute hole, nearly of a trian- 
gular form, and just of a sufficient size to” 
receive the teat, to which the little creature 
adheres so firmly, that it is scarcely matter of 
surprise that Beverly* and other writers have 
asserted that the young grow fast to the teats. 
“ Tt is not true that the young cannot 
detached from the mother without the loss of 
blood ; I can assert the contrary from many ex- 
periments made upon embryons weighing nine 
grains and upwards. I have fully satisfied 
myself as to all the various circumstances, both 
in the structure and in the exertions of the — 
minute animal, which enable it, while yet a 
mere speck of living matter, to cling so firmly — 
to the fountain of its support. ‘ 
¥ The wonderful little Didelphis is 
by no means the inanimate or the passive being 
~ 
some physiologists and naturalists have repre- 
sented it.t ‘a 
“ The toes of the ne of ve new born 
embryon opossum are furnished with sharp and 
hard waite of claws, but this is not the case 
with the hind-feet. The latter are for some 
weeks of little use to the animal; but by means 
of the former it is enabled to cling most firmly 
to the teat, and especially to the hair in the 
marsupium immediately around the teat. 
An opossum-embryon, or fetus, which 
weighed sixty-seven grains, lived upwards of 
thirty hours after I had detached it from the 
teat. Another which weighed 116 lived thirty. 
eight hours, at which time I killed it by putting 
it in spirits. , 
“* Superfcetation (I should perhaps in st 
propriety say uterine superfcetation) is wholly 
incompatible with the established laws of the 
economy of the Didelphis. But Nature, always 
provident, wastes no time. While, therefore, 
the first litter of young opossums are fast ai 
proaching to their adult state, the mother ac 
cepts the ardour of the male; she is impreg 
nated ; and after a gestation which is not, — 
think, remarkably short, if we consider 
small size of the embryons when they are 
cluded from the uterus, the marsupium is d 
tined to perform the office of a second, I was 
going to say a more important, uterus; just at 
the time when the first litter have attained su 
a size that they are no longer (one or two 0 
* History of Virginia, p. 136, 1722. 
t Pennant, Arctic Zool, i, p. 84. 
