zero population growth (ZPG) equilibrium condition in which the human population is neither increasing nor decreasing in size because the birth rate equals the death rate zona pellucida protein layer ...zero population growth (ZPG) equilibrium condition in which the human population is neither increasing nor decreasing in size because the birth rate equals the death rate zona pellucida protein layer surrounding the cell membrane of a human egg zoonosis (plural, zoonoses) infectious disease of nonhuman animals that can be transmitted to humans zygote diploid cell that forms when two haploid gametes unite during fertilization
