ORGAN DEVELOPMENT IN VERTEBRATES 



277 



Young limb-buds of the chick can be excised from their normal site 

 and planted into the coelom just lateral to the somites, where they will 

 continue their development in a remarkably normal manner. In such 

 situations the hmb-buds are often very incompletely innervated. In the 

 nerveless limbs the muscles atrophy nearly completely, and the growth 

 rate is somewhat less than normal, but although no movement at the 



v\ 



Figure 12.13 



On the left, two stages (in section) of the Hmb-bud of the chick, showing 

 the presumptive areas. C, coracoid; S, scapula; oblique shading, upper arm: 

 dashes, forearm; horizontal shading, wrist and hand; A, apical ectoderm. 

 On the right, the results of excising anterior [A above) or posterior (B 

 below) regions of the apical ectoderm; the parts developing are shaded. 

 (After Saunders 1948.) 



joints occurs, the structure of the skeletal elements is exceedingly normal, 

 even to the formation of the joint surfaces (Hamburger 1939) (Fig- 12. 14). 

 It is well known that if, owing to injury, a functional limb has to be moved 

 in an abnormal manner, the joint surfaces may become modified in such 

 a way as to facilitate this. It is clear, then, that function can play a part in 

 moulding the structure of the skeleton, but the development of these 

 isolated and functionless limbs shows that this structure can also develop 

 with a considerable degree of perfection, solely under the influence o{ 

 factors inherent within the developing tissues themselves. The interaction 



