Final answer:
During the sixth week of embryonic development, the upper and lower limb buds go through significant transformations, including the appearance of the limb ridge, formation of handplates or footplates, and the beginning of limb rotation, all critical for future limb movement.
Step-by-step explanation:
During the sixth week of embryonic development, significant changes occur in the formation of the upper and lower limb buds. The limb movement and shaping events can be summarized as follows:
- (a) The first appearance of the upper limb bud, also known as the limb ridge, is visible.
- (b) The flattening of the distal limb to form the handplate or footplate areas can be observed.
- (c) The initiation of limb rotation, an important step in the proper orientation of the limbs.
This process ultimately leads to the complex development that allows for the fine motor skills associated with limb movement, including the many intricate movements of the fingers and thumb which are critical for actions such as grasping or texting.