. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 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. 211C). 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 (
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 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. 211C). 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 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Nelsen, Olin E. (Olin Everett),
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