676 PROFESSOR OWEN ON THE MARSUPIAL POUCHES, MAMMARY GLANDS, 
is associated with so great a development of the inverted tegumentary fold, as to make 
the mammary relation seem a very subordinate and reduced one. But in the Marsu- 
pial series there is a gradation ; and both in Thylacinus and in the small dorsigerous 
Opossums of South America ( Didel'phys dorsigera, D. murina , D. pusilla , &c.), the mar- 
supial structure, if shown at all, is represented by a pair of shallow semilunar fossse, 
with their concave outlets opposite to each other, as in Echidna. 
In this comparison the distinctive peculiarity of the parts in the terrestrial Mono- 
treme is the absence of a teat, or of any rudiment of such : no part of the fundus of the 
pouch is again everted, produced, or folded about the terminal ducts of the mammary 
gland, so as to form a pedicle by which the young could take hold with the mouth, and 
so suspend itself and suck. 
The question remains, whether the marsupial pouches of the Echidna increase with 
the growth of the young 1 It is certain that they commence with the growth or 
enlargement of the mammary glands preliminary to birth. 
In that young specimen of female Echidna in which the glands were first discovered*, 
their ducts opened upon a plane surface of the abdominal integument. In a nearly 
full-grown unimpregnated female, preserved in spirits, which I examined and com- 
pared with the breeding mother here described, there is also a total absence of inflected 
folds of the integument where the mammary ducts terminate. 
Some movement, perhaps, of these ducts in connexion with the enlargement of the 
mammary lobes, under the stimulus of preparation for a coming offspring, may, with 
associated growth of the abdominal integument surrounding the areola, be amongst 
the physical causes of the first formation of the pouch. 
It has already been remarked that the integument of the pouch, especially as it 
approaches the fundus, is thinner than that covering the abdominal surface of the 
body, from which the pouch is continued. Such tegumentary growth, continued with 
the pressure of the part of the growing young within, may lead to a marked increase 
of size ; to he reduced, perhaps, by absorption and shrinking of the skin concomitantly 
with reduction of the mammary glands after the term of lactation has expired. I much 
doubt, however, whether the increase of size of the pouch would ever be such as to 
include and wholly conceal the young animal ; it more probably, at the later period of 
lactation, serves only to admit the head or beak. Thus the ordinary condition of sucking 
would be reversed in these Australian Mammals ; instead of the excretory ducts on 
an everted process of integument being taken into the mouth, this is received into an 
inverted pouch into which the milk is poured. 
I have not hitherto met with any trace or beginning of such abdominal pouches in 
the various Ornithorhynchi in which I have had occasion to note different phases of the 
development of the ovaria and mammary glands f. 
* Philosophical Transactions, 1832, p. 537, PI. XVII. figs. 2 & 3. 
t “On the Mammary Glands of the Ornithorhynchus jparadoxus ,” Philosophical Transactions, 1832, p. 517. 
PI. XY.-XYIII. 
