248 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



in the first period of transplantation, while subsequently the transplant adapts 

 itself to the growth rate of the host; this was observed after transplantation 

 of the heart primordium from Ambly stoma tigrinum to Ambly stoma punc- 

 tatum; moreover, here the more rapid growth of the transplant in the first 

 period was accopanied by a more rapid differentiation. 



On the other hand, it has been observed by Detwiler that when parts of 

 spinal cord are transplanted from Amblystoma tigrinum to Amblystoma 

 punctatum, the transplant not only grows better than the corresponding 

 organs in the host tissue, but even better than the non-transplanted donor 

 organ in Amblystoma tigrinum. Similar observations were made in the case 

 of limb transplantations. It is possible that in this instance differences in the 

 organismal differentials between host and transplant exerted a stimulating 

 effect on the graft. However, this stimulation of growth following hetero- 

 transplantation of cord tissue again applies only to an early period; subse- 

 quently, an adaptation takes place between the size of the transplant and the 

 corresponding organs in the host. The experiments of Detwiller regarding 

 the factors regulating the growth of the nervous system prove that the out- 

 growth of nerves was not determined by species-specific substances. 



The age of the host affects the transplant in a characteristic way. If an 

 eye of a young organism is transplanted to an older host, its growth is accel- 

 erated, so that its stage of development after some time is equal to that of 

 the host, while an older eye, having attained a more advanced stage of de- 

 velopment, after transplantation to a younger host grows more slowly, so 

 that the eye of the host, after some time, reaches the same stage of develop- 

 ment as the transplant. Twitty explains these phenomena on the basis of 

 Robb's specific partition coefficients for foodstuffs which different tissues 

 possess, a theory related to the conception of athrepsia of Ehrlich. However, 

 if differences in partition co-efficients, inherent in different tissues and chang- 

 ing in accordance with the ontogenetic stage of development, should be 

 responsible for these results, this would presumably be a factor of only 

 secondary importance, the primary factor consisting in differences in the 

 inherent growth energy of various tissues, upon which would depend the 

 amount of foodstuffs which the various tissues attract and use. The influence 

 of age on the growth energy of the transplant appears to be similar to the 

 effect which the time of metamorphosis has on the growth of transplants in 

 urodele larvae and which will be discussed later. 



In these more primitive organisms, such as larvae of urodele and anuran 

 amphibia, there is some indication that relatively undifferentiated cells remain 

 preserved through certain periods of larval life and that it is these cells 

 which in ontogenetically more primitive organisms give origin to a blastema 

 endowed with great regenerative potency. The presence of such cells would 

 also account for the transformability of relatively primitive transplants under 

 the influence of host tissues acting as organizers in a certain "action field" 

 of the host. It may perhaps be assumed that in urodeles such less differentiated 

 cells remain preserved longer than in anuran larvae and in this way the 

 greater regenerative power of the former may be explained. These cells are, 



