,Qg PRINCIPLES OF EMBRYOLOGY 



is removed from a limb, which is then amputated through the defective 

 region the regenerate wiU be provided with bone, although the bone 

 will not be replaced in the region of the old limb from which it was 

 excised (but this situation does not seem to hold for the tail, smce if the 

 axial organs are removed from that organ, amputation is foUowed by the 

 regeneration of a structure with the same defect as the stump [Vogt 193 il)- 

 Finally, Schotte (1940) has claimed that if a very young regeneration 

 blastema is transplanted into a situation in which it is exposed to 

 the appropriate mducers, it may be caused to differentiate mto lens or 



ear tissue. , 1 1 • ^ 



We seem therefore to be driven to the rather unexpected conclusion 

 that, although the cells of the early blastema are very labile m their 

 histological properties, and can become almost any tissue (except prob- 

 ably nerve), there is little evidence that they can change their organ- 

 specificity. If this is so, then, for instance, tail epidermis can ^^dergo a 

 'metaplasia' by which it becomes converted mto muscle-but it wiU be 

 tail muscle even if the process occurs after transplantation to a site on the 

 limb. Since organ-specificity is a rather unfamihar concept and we have 

 no clue as to its chemical basis, it is surprising to fmd it obtrudmg itself 

 in such a defmite mamier m experhnent. There are, however other 

 contexts in which it appears that organ-specificity is a rather distinct 

 character in the later stages of development. For instance, m birds the 

 mesodermal core of a feather papilla can mduce epidermis to develop 

 into a feather germ; and the type of feather eventually produced depends 

 strictly on the region of the body from which the competent epiderims 

 comes, breast epidermis always forming breast feathers, saddlee pidermis 

 saddle feathers and so on (Wang 1943)- 



The evidences of Hstological metaplasia are of fundamental import- 

 ance for our understanding of the process of determination. They show 

 that determination which occurs m embryonic stages, and the high degree 

 of histological differentiation which follows it, need not involve absolutely 

 irreversible changes, although shortly after the period of embryonic 

 determination no means are known to bring the cells back to a plastic 

 condition. Later m life the stimulus of woundmg may have tbs effect. 

 It follows that those nuclear genes which are concerned m the differentia- 

 tion of the new histological type to which the cells switch over must 

 have persisted throughout the earlier period. If the imdifferentiated blas- 

 tema ceUs are capable of redifferentiating into any and all adult tissues 

 one would have to conclude that the whole of the genotype was still 

 available and that there had been no irreversible mactivation of genes 

 during development. The evidence does not yet go quite so tar as this, 



