3l6 PRINCIPLES OF EMBRYOLOGY 



are more moderate, only the hypostome can induce a new hydranth in 

 this way. hi Tubularia, on the other hand, as we have seen, mere lacera- 

 tion, or the injection of a bubble of oxygen, will suffice to produce the 

 same result, hi such extremely regenerative forms, the tissues can be cut 

 up fmely and reduced to a homogeneous mass, and are still capable of 

 giving rise to a well-organised hydranth from the upper surface where 

 the availability of oxygen is greatest (cf. Barth 1940). 



Figure 14.3 



Frequency of interstitial cells along the stem o£ Tubularia. (After Tardent 



1952.) 



It is worth noting, as a curious and so far unexplained fact, that a hy- 

 dranth which is actively in the process of regeneration is not able to re- 

 store itself if injured before the process is complete (Davidson and 

 Berrill 1948). The regenerative capacity appears rather suddenly just when 

 the new hydranth reaches its final functional histological state; and it 

 will then not only react by regeneration to any subsequent injury, but 

 will replace parts which have been removed during its development. 

 This may perhaps mean that during the first regeneration all the neo- 

 blasts are involved in the process and there are no more available to deal 

 with a second injury, but the matter has not been fully investigated. It is 



