DEVELOPMENT OF THE SKIN 



589 



the center of the thickened area moves downward to a greater extent than the 

 margins. Some disintegration of the central cells also occurs. As a result, a 

 slight cavity or crater-like depression is formed in the middle of the epithelial 

 mass of the rudiment (fig. 211 C). In the cow and rat, this depressed area 

 continues in this state, while the edges of the cavity and adjacent integument 

 grow outward to form the nipple (fig. 277E). This type of nipple is called 

 an inversion nipple. The ducts of the gland thus open into the bottom of the 

 nipple (teat or mammilla). In the human, the original depression and the 

 openings of the primary ducts of the gland gradually are elevated outward 

 to form the type of nipple or mammilla indicated in figure 211 A. This type 

 of nipple is called an eversion nipple. 



Fig. 277. Diagrams showing mammary-gland development. (A) Human nipple 

 showing mammary duct openings. (Modified from Maximow and Bloom, A Textbook of 

 Histology, after Schaffer. 1942, Saunders. Phila.) (B) Transverse section of early nipple 

 rudiment of 20-mm. pig embryo. (C) Transverse section through developing nipple of 

 pig embryo of 70 mm., showing epidermal invagination into the dermal area of the skin. 

 (D) Section through nipple of mammary gland of human male fetus, eight months old. 

 (After Pinkus, Keibel and Mall: Manual of Human Enihryology. Vol. I, 1910, Lippincott, 

 Phila.) (E) Section through developing nipple of newborn rat. (Redrawn and modified 

 from Myers, '16. Am. J. Anat., 19.) (F) Development of human mammary gland from 

 birth to maturity. 



