THE ORGANS OF THE OUTER GERM-LAYER. 



529 



g 



I present at the very beginning of the discussion the following 

 proposition, which is of importance in interpreting the conditions 

 found : each milk-gland in Man is not a simple organ, like an ear- 

 gland or a submaxillary salivary gland, witli a, simple outlet, but 

 a great glandular complex. Its earliest fundament has been 

 observed in the human embryo at the end of the second month as a 

 considerable thickening of the epidermis (fig. 295) upon the right 

 and left sides of the breast. It has arisen as the result of a special 

 proliferation of the mucous layer, which has sunk into the derma 

 in the form of a hemispherical knob (df). But modifications arise 

 afterwards in the corneous layer also, by its becoming thickened and 

 projecting as a corneous plug into the proliferation of the mucous 

 layer. Ordinarily there is found 

 a small depression (g) at the 

 middle of the whole epithelial 

 fundament. 



The proliferation of the epi- 

 dermis that first appears is not 

 precisely, as assumed by REIN, the 

 first fundament of the glandular 

 parenchyma ; it therefore does 

 not correspond to the epithelial 

 plugs which sink into the derma 

 in the development of the sweat 

 and sebaceous glands, because 

 the further course of develop- 

 ment and especially comparative- 

 anatomical studies show, that by 



the thickening of the epidermis there is only an early delimitation 

 of a tract of the skin, which is subsequently metamorphosed into the 

 nipple-area and papilla, and from the floor of which the separate 

 milk-producing glands at length sprout forth. 



The correctness of this view is shown by the following changes : 

 In older embryos the lens-shaped thickening produced by the 

 proliferation of the epidermis has increased at the periphery and 

 has thereby become flattened (fig. 296 df). At the same time it is 

 more sharply defined at the surface, owing to the derma becoming 

 thickened and elevated into a wall (dw) the cutis-wall. Therefore 

 the whole fundament now has the form of a shallow depression (df) 

 of the skin, for which the name gland alar area is very appropriate. 

 For there early grow out from its mucous layer into the derma solid 



34 



Fig. 295. Section through the fundament of 

 the milk-gland of a female human embryo 

 10 cm. long, after Hi'ss. 



df, Fundament of the glandular area ; g, small 

 depression at its surface. 



