Deciduous teeth or primary teeth, also informally known as baby teeth, milk teeth, or temporary teeth, are the first set of teeth in the growth and development of humans and other diphyodonts, which include most mammals but not elephants, kangaroos, or manatees, which are polyphyodonts. Deciduous teeth develop during the embryonic stage of development and erupt (break through the gums and become visible in the mouth) during infancy. They are usually lost and replaced by permanent teeth, but in the absence of their permanent replacements, they can remain functional for many years into adulthood. Development Formation Primary teeth start to form during the embryonic phase of human life. The development of primary teeth starts at the sixth week of tooth development as the dental lamina. This process starts at the midline and then spreads back into the posterior region. By the time the embryo is eight weeks old, there are ten buds on the upper and lower arches that will eventually become the primary (deciduous) dentition. These teeth will continue to form until they erupt in the mouth. In the primary dentition, there are a total of twenty teeth: five per quadrant and ten per arch. The eruption of these teeth ("teething") typically begins around the age of six months and continues until 25–33 months of age during the primary dentition period. Usually, the first teeth seen in the mouth are the mandibular central incisors and the last are the maxillary second molars. The primary teeth are made up of central incisors, lateral incisors, canines, first molars, and second molars; there is one in each quadrant, making a total of four of each tooth. All of these are gradually replaced by similarly named permanent counterparts except for the primary first and second molars; they are replaced by premolars. The erupting permanent teeth cause root resorption, where the permanent teeth push on the roots of the primary teeth, causing the roots to be dissolved by odontoclasts (as well as surrounding alveolar bone by osteoclasts) and become absorbed by the forming permanent teeth. The process of shedding primary teeth and their replacement by permanent teeth is called tooth exfoliation. This may last from six to twelve years of age. By age thirteen, there usually are only permanent teeth remaining. However, it is not extremely rare for one or more primary teeth to be retained beyond this age, sometimes well into adulthood, often