Mammary gland gestation, serum progesterone remains at a stably high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnts secreted from mammary epithelial cells act paracrinely to induce more neighboring cells' branching. When the lactiferous duct tree is almost ready, "leaves" alveoli are differentiated from luminal epithelial cells and added at the end of each branch. In late pregnancy and for the first few days after giving birth, colostrum is secreted. Milk secretion (lactation) begins a few days later due to reduction in circulating progesterone and the presence of another important hormone prolactin, which mediates
Birth deriving their nourishment from a mammal-like placenta attached to the inside of the mother's uterus. In a recently described example, an African species, "Trachylepis ivensi", has developed a purely reptilian placenta directly comparable in structure and function to a mammalian placenta. Vivipary is rare in snakes, but boas and vipers are viviparous, giving birth to live young. The majority of insects lay eggs but a very few give birth to offspring that are miniature versions of the adult. The aphid has a complex life cycle and during the summer months is able to multiply with great rapidity. Its reproduction is