1. The difference in time existing between the first shedding of the skin and the reduction of the gills to mere stubs without fringes is constant and unchangeable, which indicates that the fundamental cause for both is a common one.
2. This common cause is the action of iodine, and consequently both phenomena constitute, or at least are part of, the metamorphosis of the salamanders.
3. The development of the adult skin coloration and of the legs may take place either before or after metamorphosis. Iodine cannot enforce either of these phenomena.
4. The same is true of the development of the sex organs.
5. Development of the tongue and palatal teeth can be checked even in animals in which metamorphosis takes place.
6. Consequently development of the skin coloration, as well as development of the legs, sex organs, tongue, and palatal teeth are all caused by substances not identical with the substances causing metamorphosis and, since they are also all independent of each other in their development, it is probable that special chemical mechanisms exist for the development of each one of these six groups of organs.
7. This assumption is also supported by the fact that the order of development in several of these organ pairs can be changed by a difference in temperature, which would indicate that the development of each of these groups of organs is caused by chemical reactions with different temperature coefficients.
8. That the germ cells can develop in amphibians either before or after metamorphosis does not mean that the germ plasma is opposed as a unit to the somatic plasma, since other organs which are believed to be part of the somatic plasma behave in this respect like the germ cells.
9. The noteworthy feature of the amphibian metamorphosis is that instead of being controlled and kept in harmony by the organic individual the development of at least six groups of organs is controlled separately by the action of probably six different chemical mechanisms, each of which can be stopped or enforced independently either by directly supplying the substances required or by causing an increased formation within the body by suitable temperatures.