Cartilage differentiation in the upper limb bud of the white mouse (NMRI) begins in the humerus anlage on day 12 + 13 h p.c.; cartilage differentiation in the phalanges begins on day 13 + 3 h p.c. Morphologically, differentiation can be characterized as follows: (1) cell densifications with an increase in contact structures between the cells in the blastema centre; (2) development of a two-layered cell densification: a central cartilage zone and a peripheral zone (perichondrium), between precartilage and prospective muscle anlage; numerous mitoses appear in this peripheral zone; (3) appearance of filaments and PG granula in the intercellular space as well as massive appearance of rough endoplasmic reticulum in the cytoplasm and, (4) increased distance between the cells coupled with the loss of specific contact structures and substance increase in the intercellular space.