Final answer:
Arnold Gesell observed many children to determine motor development milestones, which are the average ages that most children reach specific physical developmental stages.
Step-by-step explanation:
Arnold Gesell was a noted psychologist and pediatrician known for his work on the development of children. From his extensive observations of many children, Gesell determined motor development milestones. He documented the average ages at which most children reach specific developmental milestones in areas such as crawling, walking, writing, and dressing. These milestones help in understanding how children develop physically over time and provide guidelines to compare the development of children with their same-age peers.