PREFUNCTIONAL AND FUNCTIONAL PERIODS 



419 



During the early stages of development, when the whole organ- 

 ism or its major organ-systems are still in the gradient-field con- 

 dition, removal of a small portion of tissue will not result in the 

 absence of any particular structure, for regulation is possible within 

 the gradient-field. At this stage, no structures have been locally 

 determined, and loss of tissue does not imply loss of any definite 

 rudiment. It is only later, during the mosaic stage of development 

 when the various rudiments are chemo-differentiated, that regu- 

 lation is impossible. Later on, again, the power of regeneration 



Fig. 201 

 Correlation of size with rate of development in fore-limb rudiments of the 

 axolotl. The very early limb-bud of one embryo is removed and superposed on 

 the mesodermal portion of the limb-bud of a host embryo of the same stage. The 

 two rudiments fuse to produce a single enlarged limb, in which differentiation 

 is more advanced than in the normal limb of the other side. Top: the host 

 limbs; left, unoperated normal limb; right, experimentally enlarged limb with 

 larger digit-rudiments and more advanced skeletal condensation. Below: the 

 limbs of the donor; left, unoperated normal limb; right, small limb-rudiment 

 regenerated from the remainder of the limb-field. (From Filatow, Zool. Jahrb. 

 (Abt. allg. Zool. Physiol.), li, 1932.) 



appears. Regeneration, as pointed out by Przibram,^ is intimately 

 bound up with growth, and the onset of the capacity for regeneration 

 after the mosaic stage of development is connected with the onset 

 of the capacity for growth at this stage. Regulation and regenera- 

 tion must therefore be carefully distinguished, since they involve 

 developmental processes which are very different, and are operative 

 at different periods of the life-cycle.- 



Regeneration also, in some cases at least, appears to be con- 

 nected with the development and function of the nervous system. 



^ Przibram, 19 19. 



27-2 



