The vital link that ensures continuity of species between organisms of one generation and the next generation is-

zygote

The formation of the diploid zygote is universal in all sexually reproducing organisms. In organisms with external fertilization, the zygote is formed in the external medium (usually water), whereas, in those exhibiting internal fertilization, the zygote is formed inside the body of the organism.

Further development of the zygote depends on the type of life cycle the organism has and the environment it is exposed to.

Zygote is the vital link that ensures continuity of species between organisms of one generation and the next. Every sexually reproducing organism, including human beings, begins life as a single cell–the zygote.