112 DIFFERENTIATION 



These examples are all taken from ontogeny. The study 

 of Regeneration will give a few more. Thus in earthworms, 

 new structures heads and tails are developed over the cut 

 ends of the nerve-cord, and by exposing more than one cut 

 surface of this part, the number of regenerated heads or tails 

 may be correspondingly increased. So in the regenerating 

 tail of vertebrata (Urodela). Injury to the nerve-cord starts 

 a new formation, the cut end of this structure exerting appa- 

 rently some stimulus upon surrounding tissues. 



When aneurogenic limb buds of Amphibian embryos i.e. 

 limb buds removed before the ventral root (motor) nerves 

 have grown into them are transplanted on to strange posi- 

 tions in normal individuals, the posterior limb bud for instance 

 on to the back of the head of another tadpole, they continue 

 their development and acquire a normal nerve-supply to their 

 normally differentiated muscles ; the nerve-supply is derived 

 from the nervous system of the host, the fibres growing into 

 the graft and being guided in their course by the developing 

 muscles of the latter (Harrison). The directive stimulus so 

 exerted by one part upon another is seen again in the rege- 

 nerating appendages of Insect larvae (Agrionidae), where the 

 joints of the exoskeleton only appear after the insertion of 

 the tendons of the newly differentiated muscle fibres. 



Though the instances which at present can be cited of this 

 interaction of parts to develop new structures in ontogeny 

 and regeneration are lamentably few, they at least indicate 

 the existence of a factor of the utmost importance ; for it is 

 clear that with this factor the whole process of development 

 must be immensely simplified, though of course as truly 

 determinate as if there were in the gerrn a unit representative 

 of each separately inheritable character. 



