THE ORGANS OF THE OUTER GERM-LAYER. 531 



now becomes flattened until it lies in the same niveau with 

 the surrounding skin. It is distinguished from the latter by 

 its redder color, which is due to its greater vascularity and the 

 thinner condition of its epidermis. Then during the first years 

 after birth the middle of the glandular area, together with the 

 outlets (ductus lactiferi), which there open out close to one another, 

 is raised up and becomes the nipple, in the derma of which non- 

 striate muscle-fibres are formed in great numbers ; the remaining 

 part of the area as far as the cutis-wall becomes the areola mammae. 

 The metamorphosis takes place somewhat earlier in the female than 

 in the male. 



Soon after birth alterations take place in the still feebly developed 

 glandular tissue. There occurs a transitory swelling of the pectoral 

 glands accompanied with increased blood-pressure, and it becomes 

 possible to press out of the gland a small quantity of a milky fluid, 

 the so-called witches' milk. According to KOLLIKER its formation is 

 due to the originally solid ducts of the glands acquiring at this time 

 a lumen by the fatty degeneration of the central cells, which are 

 dissolved, and, suspended in a fluid, are discharged from the ducts. 

 According to the investigations of BARFURTH, on the contrary, the 

 so-called witches' milk of infants is the product of a genuine tran- 

 sitory secretion, and is like the real milk of the mother both in its 

 morphological and chemical components. 



After birth great differences arise between the two sexes in 

 the condition of the milk-glands. Whereas in the male the 

 glandular parenchyma remains stationary in its development, 

 in the female it begins to increase, especially at the time of 

 sexual maturity and still more after the beginning of pregnancy. 

 From the first-formed ducts of the glands there grow out 

 numerous lateral, hollow branches, which become covered with 

 hollow vesicular glands (alveoli) lined with a single layer of 

 cylindrical epithelium. At the same time there are developed 

 in the connective tissue, between the separate lobules of the 

 gland, numerous islands of fat-cells. In consequence the region 

 at which the complex of milk-glands has been formed swells into 

 a more or less prominent elevation, the mamma. 



SUMMARY. 



1. The development of the hair is inaugurated in human embryos 

 by the growing down of processes of the mucous layer of the 

 epidermis- the hair^germs-^-into the underlying derma. 



