108 INDUCTION AND ORGANISATION 



surrounding tissues of the host. This will vary according to the 

 place and time of grafting, so that divergences in different- 

 iation will occur. By grafting similar cell groups into various 

 parts of hosts of different nature and age, the range of possible 

 differentiations of the material can be studied. 



The differentiation potencies of various parts of young 

 amphibian gastrulae have been studied by these methods. The 

 following results were thereby obtained. The material of the 

 vegetative field has a very strong capacity for self-differentia- 

 tion already. Holtf refer (1931) cultured this material in a salt 

 solution, and found that it differentiated into vesicles with an 

 epithelial wall. The structure of this epithelium varied in accord- 

 ance with the place of origin of the explant. Sometimes it was 

 similar to the epithelium of the branchial part of the gut, at 

 other times it had the character of stomach or mid-gut epi- 

 thelium. This shows that there are qualitative differences in 

 differentiation potency even within the vegetative field. On the 

 other hand, the range of possible differentiations of this 

 material is limited. If transplanted it produces only endo- 

 dermal tissues- This proves that the cells' inherent ''differentia- 

 tion tendency'' can maintain itself, in spite of environmental 

 influences that work in another direction. 



The marginal zone material also appears to have an intrinsic 

 capacity for differentiation. In Holtfreter's explantation ex- 

 periments, it differentiated into notochord, muscle, kidney 

 epithelium, etc. Within the marginal zone, too, there are 

 qualitative differences in differentiation potencies. The tendency 

 to form a notochord preponderates in the dorsal part, that to 

 form muscular tissue in other places, etc., as proved for 

 example by Bautzmann's (1933) transplantation experiments. 

 But the mode of differentiation of the various parts of the 

 marginal zone is not yet irrevocably fixed, or "determined". 

 In Holtfreter's experiments, notochord and muscle nearly al- 

 ways arose simultaneously from the same material. If the 

 explant is large, it may even produce ectodermal tissues, as 

 was first noted by Lopashov (1935). Evidently, the marginal 

 zone material has a wider range of possible differentiation 

 than is manifest in normal development. 



