454 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



tions in the fertilized ovum. The greater the potency, the greater and the 

 more rapidly reached will be that amount of a hormone-like substance suffi- 

 cient to direct the development of the egg into certain channels. The earlier 

 the threshold is reached at which this substance becomes active, the earlier 

 and more extensive will be its influence on the embryonal development and 

 the more fundamental will be the changes produced, while a substance devel- 

 oping late and in small quantities will affect only the latest phases of embryonal 

 development and its action therefore will be less far reaching. Hence, the 

 effect of such a substance depends upon its potency, quantity, and time of 

 appearance, and also upon the character of the substratum on which it acts 

 and on the intensity and rapidity of the processes which it influences. In a 

 preceding chapter it has been pointed out that the time-factor plays a role 

 also in the interaction between organizer and recipient tissues and that these 

 time-relations may differ in the case of homoiogenous and heterogenous 

 tissues. In heterotransplantation, therefore, incompatibilities may develop 

 between the action of the organizer contained in the transplant and in the 

 recipient tissues in the host, or vice versa. There is, here, an additional 

 interesting analogy between the processes of fertilization and transplantation. 



In a somewhat similar manner, according to F. R. Lillie, the gynandro- 

 morphism which is occasionally found in birds depends upon certain quanti- 

 tative variations in the interaction between factors residing in the tissues and 

 the hormone acting upon the latter. In gynandromorphic individuals one side 

 of the animal has male and the other side female plumage, and at the same 

 time the quantity of the female sex hormone which is produced by the 

 ovary is diminished. Lillie assumes that the female sex hormone, under these 

 conditions, is able to impress upon the feathers the female characteristics 

 only if the growth rate of the feathers during their development is sufficiently 

 slow to allow the female sex hormone to become effective, otherwise the 

 feathers assume the male characteristics. Lillie observed also that the side on 

 which the male feathers developed in some birds was often hypertrophic, 

 and he assumes therefore that the rate of growth was too rapid on this side 

 to give the female sex hormone a chance to endow these feathers with 

 female characteristics. Thus they remained male on the hypertrophied side, 

 since here the threshold of reaction for the female sex hormone would need 

 to be higher on account of the more rapid tissue growth. In this case there 

 would be an interaction between intrinsic and external factors, the latter 

 being represented by hormones which, in combination with the inherent 

 properties of the recipient tissue, determine the character of the developing 

 plumage. 



Cell and tissue differentiation and loss of differentiation (dedifferentiation), 

 as well as metaplasia, present the problem as to how far factors inherent in 

 the tissues and how far environmental factors, including the inner environ- 

 ment, and, in particular, contact substances and hormones, play a role in 

 these processes. There is a strong indication that as a rule both intrinsic and 

 environmental factors are active, but in varying proportions in different 

 areas, and that with advancing development the intrinsic factors of the 



