what is mammary gland...??

The mammary glands are a system of exocrine glands and ducts, all connecting into alveolar ducts and then into the lactiferous duct in the nipple.  If you look back at Figures 5.10 & 5.11 on pages 142 - 143 of your textbook, you will see the introductory description of exocrine glands.  Mammary glands are alveolar glands (although I am not sure if they are simple branched or compound).  In Figure 5.11, you will see that when mammary glands secrete, they fit into the apocrine gland-type, meaning that they function by pinching off parts of their cells, rather than just secreted fluid by exocytosis.  The cell bits that they pinch off break open in the ducts, so that a fluid emerges from the nipple.

  Males and females both have some glandular tissue under the nipples, however, only after exposure to estrogen during puberty does this glandular tissue develop further.  As the glands develop, adipose tissue also develops around them; this combination of glands and adipose tissue are the female breasts.  However, just because the glands are there does not mean that they are active in secretion...

  Only after giving birth to an infant are the mammary glands productive in secretion.  During pregnancy, the hormones a woman experiences are different than when she is not pregnant (as you will see in the "after fertilization" web page).  These hormones cause the mammary glands to develop further-- placental estrogen gets the ductile systems of the glands to develop and placental progesterone gets the glands themselves to develop.  This added development of the mammary glands during pregnancy can cause the breasts to enlarge quite a bit.   However, the glands do not begin to secrete until after the child is born.

  Why don't the mammary glands secrete milk before the child is born, or even when we have not been pregnant at all?  The hormone prolactin is required for glandular production of milk and secretion of milk.  This hormone is only released in any quantity during pregnancy and after childbirth.  During pregnancy, the high levels of progesterone prevent the prolactin from working on the mammary glands to cause milk production.  But after pregnancy, prolactin is still available, but now it is present in the absence of progesterone.  Prolactin, without interference by progesterone, can now cause milk production and secretion.