228 



THE MOSAIC STAGE OF DIFFERENTIATION 



some of the original presumptive limb-area does not give rise in 

 normal development to limb, but merely to flank, skin, and muscle. 

 Similarly, if the 4-day leg-bud be divided longitudinally, so as 

 to separate preaxial and postaxial halves, the fragment usually 

 forms only those digits which it would have produced if left in situ 

 and either a tibia or a fibula. The femur rudiment regulates to a 

 miniature whole in each portion. The limb-bud thus appears to be 



Fig. Ill 



Self-differentiation of the femur-head joint, without function. The femur shown 

 in the section belonged to a limb from which all nerve supply was excluded. In 

 spite of the fact that the limb never functioned at all, the cartilage cells and fibres 

 of the femur-head show the normal configuration. (From Hamburger, Arch. 

 Entwmech. cxiv, 1928.) 



a thorough-going mosaic of predetermined but slightly overlapping 

 regions (fig. no). 



The recognition of the existence of organ-fields, i.e. regions 

 possessing a general determination for the production of certain 

 structures, and undergoing progressive regional specification of 

 detail, constitutes one of the major advances made in the analytical 

 study of development. The organ-fields resemble the whole organ- 



