14 THE STRUCTURE OF MAN 
formation are found in certain Marsupials (Phalangista vulpina) 
and among placental Mammals as high as Carnivora (Gegenbaur). 
The question now arises, whether the developmental stages 
of the mammary glands point to primitive conditions which in ~ 
any degree persist in the lower Mammals? An examination of 
the Monotremata shows that this may be the case; and to make 
this clear we must enter somewhat further into detail. 
In the Monotremata, in which as yet there are no teats, the 
ducts of the mammary organ open in a group on the ventral 
integument. As the reproductive period approaches, if fertilisation 
has taken place, a temporary depression of the ventral integument 
occurs, which gives rise to a pouch (i.m., Fig. 10). The egg is 
deposited in this pouch, and the mammary fluid is probably 
carried to the young animal to which the egg gives rise, by 
means of the pointed tufts of hair which project around the 
apertures of the glands. Closer examination shows that the ducts 
open into two cutaneous depressions, which he near the tufts just 
mentioned, in the lateral folds of the mammary pouch. These 
may be called mammary pits, and are of considerable importance, 
because they are repeated in the development of the various 
forms of nipples and mammary organs occurring in the higher 
orders of Mammals. We have here a glandular area which, like 
that already described (Fig 9, A), is nothing more than a de- 
pressed portion of the external integument, with all its character- 
istic derivatives, such as hairs, glands, and pigment. 
Before passing to the question of the disposition of the 
mammary glands on the body, an important discovery, for which 
we have to thank Oskar Schultze, must be mentioned. 
In young embryos of Mammals, eg. the Pig, a ridge-like 
prominence (/.m., Fig. 11) is found on each side, running from the 
base of the anterior limb, which is at this period a mere stump, 
towards that of the posterior limb and into the inguinal furrow. 
This is due to a linear thickening of the developing epidermis, 
and especially of the stratum Malpighi. This lateral epidermal 
ridge represents the common epithehal rudiment of the mammary 
glands, and may be called the “ Mammary Line.” Along this line 
a row of fusiform thickenings develop (Fig. 11, B and C), the whole 
presenting the appearance of a regularly varicose fibre. These 
protruding “primitive teats” flatten out again at a later stage, 
and in no way represent the teats which form later, although they 
generally correspond in number with the centres of origin of the 
future glands. 
