14 THE STRUCTURE OF MAN 



formation are found in certain Marsupials (Plialangista 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 (b.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 lie 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, e.g. the Pig, a ridge-like 

 prominence (l.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 epithelial 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. 



