Amphibians 



273 



quent experiments on A. punctatum and T. 

 torosus verified this assumption (Holtfreter, 

 '45b, '47b). 



Cellular adhesion and integrity of the cell 

 membrane in isolated amphibian tissues de- 

 pend largely on the pH and the presence of 

 an adequate amount of calcium ions in the 

 isotonic salt solution. A lowering of pH to 

 4.5 or the addition of glucose, sucrose or 

 histone to the culture medium usually re- 

 sulted in an epidermal differentiation of 

 A. punctatum explants, probably owing to 

 a fortification of the cell membrane. On the 

 contrary, neural differentiation was obtained 

 in gastrula ectoderm of T. torosus which had 

 been briefly treated with inorganic acids, 

 or alkali, alcohol, distilled water, or cal- 

 cium-free standard solution. These agents 

 are known to increase cell permeability; 

 they cause swelling and ameboid dispersal 

 of the cells, and finally rytolysis when ap- 

 plied for longer periods. Reintegration of 

 the dispersed cells was produced by their 

 transfer to a neutral balanced medium. The 

 closer the cells were brought to the brink 

 of death, the more pronounced was their 

 tendency to become neural. Yamada ('50a) 

 obtained similar neuralizations in isolated 

 ectoderm of T. pyrrhogaster by exposing it 

 briefly to ammonia. 



In the above experiments the prospective 

 epidermal and medullary areas of the gas- 

 trula reacted alike. Any portion of the 

 ectoderm could form brain-like structures as- 

 sociated with olfactory placodes, rudimen- 

 tary eyes, mesenchyme and pigment cells. 

 Since this pattern of cephalic structures de- 

 veloped in the absence of a structured in- 

 ductor and of any determination field of the 

 host, it must have arisen by self-organiza- 

 tion of the stimulated explants. The absence 

 of a locally applied inductor expressed itself 

 in the fact that the brain diverticula were 

 not bilaterally symmetrical but multiple for- 

 mations; they could be associated with as 

 many as twelve olfactory placodes in one 

 explant. 



It was thought at first that we were deal- 

 ing once more with a "relay" mechanism, 

 killed cells acting as inductors for the intact 

 cells. If this were the case, then all further 

 progress would have been stalled. However, 

 neuralization was also observed in the ab- 

 sence of any permanently damaged or dying 

 cells (Holtfreter, '47b). Obviously, the ex- 

 ternal stimulus liberated the inductive agent 

 within the reacting cells themselves by caus- 

 ing initial and reversible steps of cytolysis. 

 The similarity between this process in living 



cells and the "unmasking" of the neuraliz- 

 ing agent in killed cells is imderlined by 

 the fact that in either case many different 

 injuriovxs treatments can cause this libera- 

 tion. The essential and common mechanism 

 by which the cytolytic agents initiate neu- 

 ralization of the explants seems to be: (1) 

 increase of permeability of the cell mem- 

 brane; (2) flooding of the cytoplasm with 

 water and the electrolytes of standard so- 

 lution; (3) dissociations and, after recovery 

 from the shock, new combinations of certain 

 cytoplasmic compounds whose specific syn- 

 thetic activity would shift differentiation 

 from epidermal to neural. It is unlikely that 

 the initial steps of this mechanism occur in 

 normal development also. 



Some of the inductive effects discussed 

 above can conceivably be reinterpreted in the 

 light of these experiments. For instance, 

 neuralization by methylene blue, organic 

 acids, water-soluble carcinogens or sulf- 

 hydryl compounds could be due to a reversi- 

 ble cell injviry rather than to a relay mech- 

 anism. The mechanism by which certain 

 carcinogens exert their neuralizing effect 

 may be similar to the unmasking effect of 

 subcytolytic agents. Waddington and Good- 

 hart ('49) found that such a hydrocarbon 

 becomes selectively fixed to the large gran- 

 ular cell inclusions (lipochondria) which 

 may entail the liberation of the neurogenic 

 factor. 



The fact that an abnormally high water 

 imbibition of the cell, caused by unspecific 

 injurious agents, is apparently sufficient to 

 elicit neuralization demonstrates once more 

 the futility of the attempts to identify any 

 of the experimentally applied chemicals with 

 the normal neuralizing factor. On the other 

 hand, our conviction is strengthened that 

 the key to an understanding of the induc- 

 tion phenomena is to be sought in the re- 

 acting cells rather than in the inductors. 

 This supposition led to a comparative study 

 of some cytological phenomena in differen- 

 tiating epidermal and neural cells. 



CELLULAR POLARITY, MOTILITY AND 



ADHESIVENESS AS RELATED TO 



INDUCTION 



The recognition of the polar structure of 

 early embryonic cells is of fundamental im- 

 portance for an vmderstanding of neural in- 

 duction. The entire svirface of the embryo 

 is coated with a protective film having a low 

 permeability. Cellular motility is mainly 

 confined to the inner uncoated surface, 



