Embryonic nuclei, when transplanted into enucleated eggs, are usually able to direct development of tadpoles since those nuclei are already in a state (embryonic) when this is close to their normal role and fate. Adult cells' nuclei have undergone changes, such as conversion of some DNA to heterochromatin, which limits their ability to produce all of the cells in a tadpole.
Work Step by Step
The key here is to review the information here and in previous chapters which deal with changes in nuclei and chromatin. That, in turn, explains how gene expression alters with age and potential cell fates become more limited.