RELATIVE STABILITY OF LIVING MATTER 



333 



worm had been cut in the middle, both halves would have pro- 

 duced an entire worm, minus, perhaps, certain parts like the 

 reproductive organs, which do not seem to be reproduced. How- 

 ever, the posterior half of a planarian will regenerate, producing 

 an entire worm with typical eyes. 



What it is that determines the character 

 of the restored part is a mystery. A worm 

 is cut at a certain point. The tissue of 

 one piece will regenerate head with all its 

 parts, and the tissue of the other, at the 

 same point, will regenerate posterior parts, 

 unless the cut be well forward, when 

 botJi pieces will regenerate anterior parts, 

 or well back, when both will regenerate pos- 

 terior parts. 



These facts cannot be explained on the 

 theory of "formative stuffs," because head 

 tissue may arise from posterior positions 

 and tail tissue from anterior. For exam- 

 ple, if an oblique cut be made into the 

 side of a planarian, head tissue will arise if 

 the cut be directed backward, even though 

 made well to the rear, while tail tissue will 

 arise from a cut directed forward, even 

 though made at a point so far ahead that 

 if carried entirely across the body it would 

 sever a piece so small that it would give 

 rise to head tissue only (see Fig. 42). 



The determining cause pf these oppo- 

 site differentiations is yet a mystery. 



Lateral regeneration. " Since the most 

 familiar cases of regeneration are those that 

 take place at the anterior and posterior 

 ends, we not unnaturally come to think of polarity as a phenom- 

 enon connected only with the long axis of the animal, but 

 there are also many cases of lateral regeneration in which a 

 similar relation can be shown." 



1 Morgan, Regeneration, p. 43. 



FIG. 42. Polarity in re- 

 generation : regenera- 

 tion from the two 

 oblique cuts opening 

 forward will produce 

 "head stuff," though 

 one cut is far posterior ; 

 on the other hand, an 

 anterior cleft opening 

 backward produces 

 tail matter. After 

 Morgan 



