1. The review considers information from mammalian embryology relevant to X-chromosome inactivation, and from X-inactivation relevant to mammalian embryology.

2. Properties of the inactive-X, by which it may be recognized are: sex chromatin, heteropycnosis, late replication and the absence of gene product. Each of these has advantages and disadvantages in particular circumstances. In some species the X carries constitutive heterochromatin, which must be distinguished from the facultative region.

3. The time of X-chromosome inactivation can be estimated from the time of appearance of sex chromatin or late replication, or inferred from the appearance of heterozygotes for X-linked genes or of experimental chimaeras. The estimated time varies with species, and in the mouse and rabbit is near the time of increase in RNA synthesis.

4. Whereas in eutherian mammals either the maternally or the paternally derived X may be inactivated in different cell lines, in marsupials the paternal X is always the inactive one.

5. During development various factors act to distort the patterns produced by random X-inactivation. These factors include cell selection, transfer of gene product, and migration and mingling of cells.

6. There is no clear evidence that X-chromosome inactivation is not complete.

7. In female germ cells both X-chromosomes appear to be active. In male ones both X and Y appear inactive during most of spermatogenesis, although probably in early stages all X chromosomes present are active.

8. The active and inactive X-chromosomes may be differentiated by presence or absence of some non-histone protein or other polyanionic substance.

9. If the genes concerned in synthesis or attachment of this substance are on the X-chromosome then the differentiation will be self-maintaining.

10. The initiation of the differentiation requires either the attachment of different X-chromosomes to different sites, or some interaction of X-linked and autosomal genes, concerned in inducing or repressing activity. Some possible models are discussed.