THE CATALYST ENTELECHY IN DIFFERENTIATION 189 



carrier, slight change in local conditions may incapacitate the carrier; 

 or adsorption of some diffusate may block it. Then even a steady 

 supply of prosthetic group molecules, from the more stable catalytically 

 functioning organizer, would fail to form the enzyme normally ex- 

 pected. Experimentally, competence would disappear. 



By implanting the embryonic dorsal lips of a toad (Bombinator) 

 into the blastococle cavity of a newt (Triton), Geinitz 10 obtained 

 inductions, thus proving that the primary evocator is not species- 

 specific. Subsequently many other mixed implantations led to success- 

 ful inductions, e.g., chick in newt, (Triton), chick in rabbit, rabbit in 

 chick, zebra fish (Danio rerio) in newt (Triturus torosus), lamprey in 

 newt (Molge). Then Holtfreter 11 made the surprising discovery that 

 the evocator exists not only in the developing embryo, but is also 

 present in all adult tissues of all family groups (phyla). Tests included 

 tissues from two worms, two molluscs, three insects, a crustacean, two 

 fishes, three amphibia, a reptile, two birds, two mammals, and man. 

 Some of these adult tissues manifested inductive capacity without 

 special treatment (e.g., boiling, protein denaturents). Even pieces of 

 the coelenterate Hydra viridis, which develops no neural axis, showed a 

 feeble but positive inductive effect on amphibian blastoceles. It has 

 also been reported 12 that implantation of blastomeres from the four- 

 cell stage will induce a neural tube in a competent (or receptive) 

 ectodermal ball.* 



Needham believes that the evocator exists in masked or inactivated 

 form in all parts of the embryo except in the organizer region. Mask- 

 ing and inactivation are terms which express results, not mechanisms; 

 but they may be the consequences of the adsorption, or the aggregation, 

 of specific substances which would thereby become incapable of diffu- 

 sion, until they were eluted, or deflocculated as explained below. 

 Such specific substances might then exhibit electronic fields having 

 catalytic power, or they could then serve as units needed to complete 

 such fields. Adsorption or chemical fixation of specific units may also 

 result in inhibition, masking or inactivation of the adsorbent or the 

 fixing units. 



O. Mangold and H. Spemann 13 found that in the course of the 

 determination and histological differentiation of the neural tube, 

 it develops the power of inducing another neural plate when 

 implanted into the blastocele cavity of another embryo (termed 

 homoiogenetic induction). As Needham 9a states: "Its cells acquire 

 induction power, and in later development the evocator appears 



* A dispersion of 7-day chick embryo extract in Tyrode solution produces induc- 

 tions most nearly approaching those of a living amphibian organizer. To Dr. Lillian 

 Baker of Carrel's laboratory fell the main task of keeping alive the bit of chicken 

 heart in a continually renewed chicken embryo juice medium. 



