POLARITY IN REGENERATION 



233 



TENTACLES 



TENTACLES 



^ 



^r t W 



U B, l_5 



BASE 



— i 



W HIGH O 



^ 



Fig. 157. The normal polarity of a hydroid and the reversal of polarity by 

 experimental treatment. Left: A hydroid is cut into two equal parts. Each part 

 retains the polarity of the original hydroid and regenerates a base and a set of 

 tentacles. Right: The end T 1? which would normally form a set of tentacles, is 

 treated with a low concentration of oxygen and forms a base; the end B l5 which 

 would normally form a base, is treated with a high concentration of oxygen and 

 forms a set of tentacles. Polarity is reversed by other chemical treatments. 



polarity of regenerating parts is not fixed. We can change it by chemical 

 treatment. 



A simple way of changing polarity in the hydroid body is to treat the two 

 ends of a polarized segment with different amounts of oxygen. If we place the 

 tissue in a chamber which is divided into two parts, we can put a high con- 

 centration of oxygen in one of the chambers and a low concentration in the 

 other (Fig. 157) . We shall label the original polarity of the part by using the 

 letters T and B to indicate the ends where the tentacles and base would be ex- 

 pected to form. It turns out that if one end is treated with a low concentration 

 of oxygen and the other end with a high concentration of oxygen, the polarity 

 is completely reversed, and the tentacles form where a base would normally 

 develop. Obviously, the organization within this tissue is labile; it can be re- 

 versed by the application of different concentrations of oxygen at the two 

 ends. 



Another method of illustrating the lability of the hydroid is the separa- 



