The answer to these questions has come from two different types of experiment — Tissue Culture and Grafting. Before we delve into the subject above, we must first understand what Tissue Culture is by asking the pertinent question: What is Tissue Culture? In Tissue Culture a small piece of presumptive tissue, say ectoderm, is isolated from the blastula and the cells kept alive in a medium containing the necessary materials. In amphibians each cell has a sufficient food supply for about three weeks from its enclosed yolk. In this case the medium merely has to provide water, salts and a buffer.
If conditions are satisfactory the cells of the explants (as it is called) divide and development occurs in isolation from other tissues. The results of such experiments indicate that in general the majority of presumptive tissue develops along fixed lines even when they are isolated from other tissues. As a broad generalization it can be said that presumptive ectoderm develops into epidermal tissue, presumptive mesoderm into somites etc., presumptive chorda into notochord, and presumptive endoderm into gut tissues. In this last case the part of the gut formed may depend on the exact piece of presumptive endoderm isolated. So it seems that for most tissues their fate is sealed, or determined to use the technical term, as far back as the blastula stage. However, there’re is one outstanding exception to this and that is the neural plate, the region of ectoderm destined to become the neural tube. When isolated and grown in tissue culture, presumptive neural plate develops not into nervous tissue but into epidermis. It seems, therefore, that unless it is subjected to some kind of influence from neighboring tissue, the tendency is for the neural plate to develop into ordinary epidermis.
Which particular tissue in the intact embryo exerts an influence on the neural plate causing it to develop into nervous tissue? This is where grafting experiments come in. by careful surgery it is possible to cut out small pieces of presumptive tissue from one embryo (the donor) and graft them into another (the host). This has been done mainly with newts and the results are tolerably clear cut. In the majority of cases the fate of the transplanted tissue is altered so that it develops into tissue appropriate to the host. Thus, for example, if a small piece of presumptive ectoderm is grafted into mesoderm of a host embryo, the graft develops not into epidermis but into appropriate mesodermal tissue. Somehow the ‘normal’ development of the graft is suppressed by the surrounding mesoderm, and its fate is changed in accordance with its new position. However, there are two situations in which precisely the reverse takes place, the donor tissue influencing the host’s tissue to develop into structures appropriate to the donor. The two tissues capable of doing this are mesoderm and prospective notochord (chorda tissue). If either of these are grafted into another tissue, the latter is transformed accordingly. It is in fact chorda tissue which causes the adjacent ectoderm to develop into nervous tissue.