Imbedding posets in the integers

An imbedding of a poset P in the integers is a one-to-one order presevring map from P into the integers. Such a map always exists when P is finite and, moreover, certain imbeddings of subsets of finite P can be extended to imbeddings of the whole of P. D. E. Daykin has asked when an imbedding in the integers of a finite subset of a countable poset can be extended to the whole poset. This paper answers Daykin's question and some related questions.