450 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



may be active also in distantly related species of insects. However, while in 

 this case the skin of the distantly related imago, if properly stimulated, still 

 possesses the ability to produce a cuticula, which ordinarily is produced only 

 by the larva, the kind of changes which take place in the skin under the in- 

 fluence of this hormone, the structure and pigmentation of the newly formed 

 cuticula, possess the characteristics of the imago skin. The modifiability of 

 this tissue under the influence of a specific hormone, obtained from a distant 

 species, is therefore restricted. However, if the skin of the imago undergoes 

 regeneration, its potentiality to react like larval skin is restored to it and 

 now the typical changes in the cuticula may be produced by the hormone. 



Similarly, Piepho has shown that a larval hormone may induce the normal 

 skin of a pupa to form the cuticula characteristic of the pupa, while regenerat- 

 ing skin regains the ability to produce larval cuticula. In the latter instance 

 the initiation of growth processes in the skin enlarges the range of reactivity 

 of this tissue to specific hormones ; when it has reached a more advanced 

 stage of regeneration it behaves like tissues of earlier embryonal stages, which 

 are as yet less differentiated; it returns to a more plastic condition in which 

 the equilibrium is more labile and in which certain changes in the inner or 

 outer environment may cause fargoing transformations. But it seems that 

 the effects of regeneration in increasing the range of reactivity of tissues de- 

 creases with increasing phylogenetic evolution, being much less in mammals 

 than in invertebrates. We have seen that the very plastic material of phylo- 

 genetically primitive organisms, such as planarians, reacts readily to environ- 

 mental changes with modifications of organs, whereas the reestablishment 

 of the original set of environmental conditions may lead again to the restora- 

 tion of the original tissue structures and tissue equilibrium, as the recent 

 experiments of Child have shown. In the very primitive and very plastic 

 material of certain coelenterates the tissue equilibrium may be determined 

 by a set of relatively simple conditions in which mechanical factors and 

 oxygen supply (Barth) may play a significant role. 



Also, in the early ontogenetic stages the as yet less differentiated tissue 

 may react to stimulation by specific hormones with tissue changes which 

 correspond more to the specificity of the hormone than of the tissue. Thus 

 in sufficiently early embryonal stages of birds (Willier) and mammals (Ivy) 

 male and female sex hormones can determine in which direction, female or 

 male, the sex glands of the embryo shall develop. 



In our experiments on the production of maternal placenta and placentoma 

 in the uterus of the guinea pig, we analyzed by means of transplantation of 

 pieces of uterus, the interaction between certain morphogenic distance sub- 

 stances and organismal differentials. We found that the formation of 

 placentomata depended upon the amount of lutein substance which has had a 

 chance to act on the uterus previous to, as well as following transplantation, 

 and action at both these periods was necessary in order to obtain the develop- 

 ment of large-sized placentomata. There entered into these reactions, further- 

 more, a mechanical, stimulating factor, which was introduced during the 

 process of transplantation. But in addition the effect depended also upon 



