Human hand anlagen of different developmental stages are studied light and 
scanning electron microscopically. The findings are compared with experimental 
and ultrastructural results obtained from avian limb anlagen. Shaping, cell 
differentiation and the spatial arrangement of different cells are found to be 
the basic processes of hand development. The shaping of the arm and hand seems 
to anticipate future grasping movements. Factors controlling this developmental 
process are on the one hand the apical ectodermal ridge (AER) that maintains in 
the underlying mesoderm a high level of mitotic activity, and on the other hand 
a species-specific pattern of cell death in different zones of arm and hand. 
Interdigital cell death, microfilament bundles included in the basal compartment 
of AER cells, and local anchorings of the AER ectoderm by collagen fibrils are 
involved in finger separation. The flexion creases are genetically fixed and 
their development cannot be explained by mechanical factors. It is found that 
the early hand anlage is already composed of relatively autonomous founder cells 
committed to different lineages. This is true for the muscle precursor cells 
which originate from the brachial somites. These migrating somite cells are 
determined to belong to the myogenic lineage. However, their distribution, 
mitotic activity and later arrangement in single muscles are controlled by 
factors localized within the hand itself. Tendons develop autonomously from 
somatopleural cells. Other already committed cells are the angioblasts forming 
the endothelial lining of the blood vessels, the neural crest cells 
differentiating into melanocytes and Schwann cells, and the blood-derived cells 
like chrondro- or osteoclasts. The differentiation of somatopleural cells into 
cartilage, connective tissue or smooth muscle depends on their position within 
the hand anlage. Possible mechanisms leading to the specific pattern are 
discussed.