46 
L. Kaufman: 
of gills remained, the first moult occurred which is the end of 
larval life. I never observed that an animal which already moulted, 
should resume again its larval form, i. e. that its fin and gills 
could regenerate. When the stimulus was strong enough to cause 
the inchoation of resorption of gills and fins, further processes of 
metamorphosis occurred independently of external stimuli. I ceased 
feeding axolotl No. 11 on thvroidine on May 16, 1917, when incipient 
reduction of the larval respiratory organs was observable and I left 
it in the deep water aquarium till July 11, 1917, when it was 
killed for histological investigation. In spite of nearly two months 
of aquatic life without thyroidine, of the larval respiratory apparatus 
only the gill-cleft remained; the gills and the fin were almost 
entirely atrophied. I lay stress on the fact that small quantities 
of thyroidine which can elicit metamorphosis provoke all changes 
leading to trasformation. 
Observations made by physiologists enable us to compare these 
morphogenetic processes with phenomena of irritability. We know 
that certain tissues or organs (the heart-muscle, the nerve-fibres) do 
not follow Weber-Fechner’s Law, that to a feeble stimulus 
above the threshold of irritability they respond by energetic reac¬ 
tion. In Physiology the process is known as the „law of everything 
or nothing“. From my observations on axolotls I conclude that this 
law may also be applied to morphogenetic processes. 
I have observed that metamorphosis, if once elicited, continues 
independently of external stimuli, even under conditions more 
favourable to the development of the larval than to that of the 
terrestrial form; hence I consider the process of transformation to 
be an irreversible one. 
Chauvin maintains that terrestrial forms of axolotls turn back 
into larvae when placed in water. On the authority of her observa¬ 
tions, the appearance of amphibians in the aquatic or the terrestrial 
form was considered a direct result of the influence of their surround¬ 
ings (C a m e r a n o, W e i s m a n n). This result would now have some 
importance for the outstanding problem of senescence and reju¬ 
venation. But Wintrebert convinced himself that axolotls which 
were made to undergo total metamorphosis were not able to 
reassume previous form. He therefore believes (and I agree with 
him) that the axolotls used by Chauvin to undergo retrograde 
changes were not yet totally transformed but had considerably 
