The three large images below show a clear plastic dentaform made by Kilgore International inc. This dentaform shows the dentition of a child of about three years old. The deciduous (baby) teeth are fully formed, fully erupted, and in occlusion (which means that the top teeth are in contact with the bottom teeth). The roots of the baby teeth are clearly visible and approximately anatomically correct. You can see the adult teeth just beginning to form in the plastic just above and below the roots of the baby teeth.
The deciduous teeth are generally fully erupted by age 2. The first teeth to erupt are the lower central incisors, and eruption proceeds from the front to the back, each pair of lower teeth preceding the corresponding uppers by two months. Some children erupt their baby teeth several months early, and some may be retarded by several months. This is normal. The adult teeth begin to form in the bone at the tips of the baby teeth.
They start forming at the incisal or occlusal surfaces (the chewing surfaces) and work their way toward the tips of the roots. Note in this view, the upper lateral incisors form medially to (behind) the central incisors, and the canines form labially to (in front of) all four. This has implications for the occlusion, since the upper canines should always erupt buccally to the lower canines in a normal class I relationship.
These two images show the relative positions of the canine, first and second premolars, and the first molar as they appear in the bone of a three year old child. The first molars form behind the second deciduous molars and are quite advanced by age three since they do not have to wait for the formation of the root of a deciduous tooth to begin their own formation. The adult first premolars generally erupt about a year before the second premolars, possibly because they form so deeply nestled between the roots of the first deciduous molars.
For a good representation of the continuous development of the teeth from before birth until adulthood, click on the illustration below.