THE CATALYST ENTELECHY IN DIFFERENTIATION 205 



Some Instances of Inductor Action 



The importance of nuclear inductors in determining developmental 

 form or morphology is brought out by the striking work of J. Ham- 

 merling 54 with the umbrella alga, Acetabularia, whose huge single 

 cell may grow to 2 or 3 cm. high, the rhizoid or root-like base having 

 a long stem terminating in an "umbrella" 0.6 cm. in diameter. The 

 umbrella is preceded by a series of wreaths of hairs, and consists of 

 a number of radial chambers which later contain the sex cells 

 (gametes). The single nucleus is in the rhizoid end, away from the 

 umbrella, but from this nucleus there diffuse substances which can 

 specifically direct regeneration of either end of the plant. Thus if 

 the plant is cut in two, the nucleated rhizoid end will reproduce an 

 umbrella. The other end, having no nucleus, may live quite a while, 

 but before eventually perishing it may regenerate an umbrella or a 

 rhizoid. The regeneration of the umbrella takes place more readily 

 the nearer the cut is to the "umbrella" end, while the opposite is the 

 case with the regeneration of the rhizoid. But in the absence of the 

 nucleus the amount of regeneration is sharply limited. 



Hammerling concluded that the nucleus produces two directive 

 substances which diffuse differentially so that more of the umbrella- 

 forming stuff accumulates toward the umbrella end, and more of the 

 rhizoid-forming stuff remains nearer the supplying nucleus. When 

 a nucleated piece was allowed to begin regeneration and the nucleus 

 was then removed, regeneration proceeded even if the cut exposed 

 part of the stem which would not ordinarily form an umbrella; for 

 the nucleus had supplied the necessary form-directing materials. 

 Their stability is indicated by the fact that if the umbrella is cut off 

 two months after removal of the nucleus, a new umbrella is re- 

 generated. If the stem is cut near the root, an umbrella will develop 

 at this unusual place; but if the nucleus is removed before completion, 

 regeneration will cease after the exhaustion of the umbrella-producing 

 substance. 



Are genes within the nucleus involved in the formation of these 

 directive substances? An affirmative answer was given by further 

 experiments with two species of the alga whose umbrellas showed dif- 

 ferences in color, in number and shape of their chambers, etc., namely 

 Acetabularia mediterranea and A. wettsteinii. On grafting anucleate 

 fragments of one species onto nucleated fragments of the other, in both 

 directions, it was found that the nature of the nucleus determined what 

 kind of umbrella would develop. Thus a mediterranea stem grafted 

 onto a nucleated wettsteinii rhizoid, gave a typical wettsteinii umbrella. 

 Goldschmidt remarks: "This shows that actually the genes within the 

 nucleus control the production of a specific formative stuff (not 



