234 



EMBRYOLOGY 



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Fig. 158. Regeneration and polarity 

 in coelenterates. Left: A section of a stem 

 is shown with the upper end, T l5 giving 

 rise to tentacles and the lower end, B 1; 

 forming a base. Right: A similar stem is 

 ligatured in the middle and as a result 

 tentacles develop at B x as well as T 1 . 

 Recall the effect of a ligature of the 

 amphibian egg, Chapter 1. 



tion of a part of the body into two parts by means of a constricting ligature. 

 This is exactly the same experimental procedure that was used on the early 

 amphibian egg. When a piece of the hydroid is tied with a ligature, both T 

 and B form tentacles (Fig. 158). Thus two individuals are obtained from 

 one mass of tissue. This result is similar to splitting an egg and obtaining two 

 embryos, or splitting a limb bud and getting two limbs from it. 



Theories of regeneration 



Now we go on to inquire about the nature of this labile organization. 

 What is responsible for it? We asked the same question regarding the or- 



