SUBSTRATUM AND MORPHOGENIC SUBSTANCES 451 



the organismal differentials of host and transplant, the transplant showing a 

 marked sensitiveness to the injurious action of homoiotoxins. In this case 

 the morphogenic substance, the lutein hormone, does not bear an organismal 

 differential and the injurious action of the homoiotoxin is due to the sensitive- 

 ness of the tissue on which the hormone acts. 



Somewhat related conditions were found in the compensatory hyper- 

 trophy of the thyroid gland, a process which in all probability is caused by a 

 change in the normal balance between the thyroid-stimulating hormone of the 

 anterior hypophysis and thyroxin, the hormone of the thyroid gland, the for- 

 mer inducing, the latter inhibiting hypertrophy. If we diminish the quantity of 

 the thyroid hormone by extirpating a part of the gland which produces it, 

 hypertrophy takes place; if we increase the quantity of thyroid hormone, 

 hypertrophy is prevented. Although these hormones do not carry homoiodif- 

 ferentials, still, homoiotransplantation of thyroid tissue interferes with the 

 development of hypertrophy, because the homoiotoxins have an unfavorable 

 effect on the graft. 



Similar problems arise in the analysis of the factors underlying meta- 

 morphosis. How far do the conditions initiating metamorphosis reside in the 

 tissues and depend upon the organ and organismal differentials of the latter, 

 and how far are they due to the action of stimulating or regulating substances 

 circulating in the bodyfluids and comparable to hormones ? It is again largely 

 by means of transplantation experiments that the analysis of metamorphosis 

 has been carried out. In his early experiments of joining together parts of 

 frog larvae, Born had observed that the two partial larvae, when they were 

 combined, metamorphosed at the same time, irrespective of the state of nour- 

 ishment of the two partners; this may be taken as an indication that one 

 partner influenced the time of metamorphosis of the other. Such an influence 

 was also noticeable in the more recent experiments of Burns, who accom- 

 plished a union between larvae of Amblystoma tigrinum, which normally 

 metamorphose very slowly, and those of Amblystoma punctatum, which 

 metamorphose more rapidly; under these conditions Amblystoma punctatum 

 caused a definite acceleration of the metamorphosis of the tigrinum larva. 



A furthergoing analysis of the factors underlying metamorphosis has been 

 accomplished through transplantation of pieces of amphibian skin and of the 

 iris of the eye, in which, normally, characteristic color changes take place 

 during metamorphosis. In this way it has been possible, within certain limits, 

 to determine how far organismal differentials influence these processes, and 

 in particular, whether an interaction takes place between the factors determin- 

 ing metamorphosis and the homoio- and heterotoxins which may act on the 

 grafted tissues. From the older experiments of Uhlenhuth on the eye, of 

 Weigl on skin, of Kornfeld on the gills of urodeles, and from the more 

 recent experiments of Lindemann on the skin of frog larvae, we may, in 

 general, draw the conclusion that a chemical factor, a substance circulating 

 in the bodyfluids of an amphibian, initiates metamorphosis synchronously in 

 all the tissues which are sensitive to the effect of such a substance and which 

 are subject to metamorphosis. Furthermore, this substance is able to act not 



