276 PRINCIPLES OF EMBRYOLOGY 



induction is much higher than it would have been if the material had been 

 left in its original location. 



Another manifestation of the field of competence can be seen in the 

 frequency with which the induction produces forelimbs or hindlimbs. 

 Forelimb-hke structures can be induced in the anterior region back to the 

 posterior margin of somite 8, while hindlimbs can be induced in the 

 posterior region, wliich extends forward to the anterior margin of seg- 

 ment 8. Thus there is a small region of overlap between the forelimb- 

 and hindhmb-producing regions. Shoulder girdles are usually not induced, 

 but the pelvis can be evoked very regularly in the posterior end of 

 the field near somites 13 and 14. These three lines of evidence — the 

 asymmetry of the induced limbs, the frequency of the induction and the 

 character of the limbs induced — clearly demonstrate the existence of a 

 field of competence in the flank, but do not show how far this is a property 

 of the localised areas of tissue, or how far it is dependent on factors which 

 act in a general way over the whole region. 



In the period immediately after the limb-bud is formed, the various 

 individual parts of it are still incompletely determined. A single bud may 

 give rise to a duphcated pair of limbs, or, in other cases, two anterior 

 half-buds may fuse to give a single limb. We shall not carry any further 

 the discussion of the gradually increasing determination of the ampliibian 

 limb-bud (see Review of Mangold 1929), but instead turn to a considera- 

 tion of the limb-buds in the chick, which illustrate certain other points 

 of general significance. 



In the chick, technical difficulties have made it impossible to investigate 

 the symmetry relations of the early limb-discs as has been done in the 

 Amphibia, nor is anything known about limb induction in birds. It is 

 in connection with the later stages of development of the limbs that 

 investigations on bird embryo have been most informative. The first 

 point we shall notice is one which emerges from the investigations of 

 Saunders (1948). He showed that the bulk of the material which forms the 

 limb-bud at the earliest stage at which it can be recognised will eventually 

 form the proximal parts of the limb. More distal parts are added later as the 

 limb-bud elongates. During this process a specially important part is played 

 by a thickened cap of ectoderm which forms the actual apex of the elongat- 

 ing bud. If this apical cap of ectoderm is removed, the laying down of fur- 

 ther distal regions of the limb ceases, and the limb remains as a stump from 

 which the distal parts are missing (Fig. 12.13). This does not occur in the 

 Amphibia, where the proximal parts can form a complete limb ; but in that 

 group, of course, proximal stumps can regenerate their missing distal parts 

 till quite a late stage, whUe such regeneration does not occur in the chick. 



