1 



A. Distal segments develop faster and better than proximal segments. 



Glass cap S^ 



Short segments form new heads 



only at the distal ends; long 

 segments, both at distal and proximal. 



C. If the distal end of a short segment 

 is prevented from forming o new heod, 

 the proximal end will do so. 



D. If the two ends of a short segment 

 are physiologically seporated, both 

 will form a new head. 



Fig. 22. Regeneration of polyps by stem segments. 



of moderate length develops a new head at only one cut end, and this is 

 invariably at the distal end. However, if we prevent the distal end from 

 regenerating by slipping a glass cap over it, then the proximal end will 

 form the head ( although slower than the distal end would have done it ) . 

 This interaction can be relieved in two ways: first, by making the stem 

 segment so long that the two ends are not in close contact; second, by 

 separating the two ends physiologically. The latter can be accomplished 

 by tying a string tightly around the middle of the segment so as to prevent 

 the flow of materials and cells between the ends. When these measures 

 are taken, heads form at both ends without any evidence of competition 

 between them. 



This type of phenomenon is commonly encountered in develop- 

 mental systems. A case in point is the development of the hind limb in a 

 chick embryo. As described in Chapter 5, the limb originates as a small 

 protuberance from a specific point along the flank of the embryo. Areas 

 surrounding this point can also be shown to be capable of forming the 



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