ABSTRACT
The causal morphological analysis of the determination and differentiation processes in early developmental stages has been successfully carried out, especially in insects, sea urchins, amphibians and mammals. In all these investigations the final outcome of isolation and transplantation experiments has been evaluated by histological analysis of the differentiated tissues. In such experiments the term determination, which has been used in several ways, can be defined as follows: A certain region of a cleaving egg or an embryo is said to be determined if after isolation it forms the same tissues as in the intact embryo.
A proteoglycan which inhibits the biological activity of the factors has been extracted from chicken embryos (Born, Tiedemann & Teidemann, 1972) and amphibian tail-bud stages.
BrCN-sepharose blocked with ethanolamine and BrCN-sepharose coupled globulin has no inducing activity.
On ectoderm of Triturus alpestris, the species most frequently used for testing tissue extracts and purified factors ‘unspecific’ stimuli have very little effect.
Whether Li+ ions activate masked internal inducers or whether they may change the conformation of chromatin directly is an open question.
The molecular basis could be a graduated differential affinity of the factor to structurally related moderately heterogeneous DNA sequences with a regulatory function.