POLARITY IN REGENERATION 231 



Regeneration in various species 



There is a great deal of variation in the lability of the adults of different 

 species. A form that shows the greatest degree of lability is the planarian 

 rlatworm. This complex organism can be split like an egg into various parts, 

 and each part will differentiate in a series of steps and regenerate a com- 

 plete organism (Fig. 155). Other organisms, such as the salamander, show 

 a more restricted ability to regenerate but nevertheless will regenerate parts 

 of the limbs or entire limbs. Thus, if the forelimb of a salamander is am- 

 putated at any level up to the shoulder, the remaining stump will grow out 

 and replace the lost part. And the replacement may include everything from 

 the upper limb region down to the digits. Figure 156 compares regenerative 

 ability in hydroids, salamanders, and frogs. 



There are other form:., such as the frog, in which this regenerative ability 

 does not normally show itself. If the limb of the frog is amputated at any 

 level the skin simply heals over the wound but no replacement of the lost 

 structure takes place. Formerly biologists considered that such a difference in 

 the ability to regenerate was correlated with the fact that animals become 

 more specialized. We shall see in the course of our studies, however, that this 

 is not an entirely reasonable interpretation of the facts. 



Polarity in regeneration 



In trying to analyze the principles operating in regeneration, we may 

 utilize the same methods that were used in the analysis of embryonic develop- 

 ment. First of all, we can show that parts of an organism have a distinct 

 polarity. As an example for part of the analysis we shall use a hydroid which 

 is similar to Hydra in having a base for attachment, a long body, and a set of 

 tentacles which take in food. If we cut such a hydroid into two parts and 

 follow the development of each of these two parts, we find that one end will 

 form the tentacles (T, Fig. 157) and the other end will simply form a basis 

 of attachment or a base (B, Fig. 157). This experiment establishes the fact 

 that there is a distinct polarity in the hydroid body. The upper end of each 

 piece forms tentacles; the lower end, a base. This polarity is similar to the 

 animal-vegetal polarity in the sea urchin egg and the anterior-posterior polar- 

 ity in the amphibian limb. And, like the polarity in embryonic systems, the 



