Answer:
Nucleation
Step-by-step explanation:
The first step of formation of new phase, structure or self-assembly is called as Nucleation.
It is mentioned in the book of Molecular Biology of the Cell that whenever, In-vitro polymerization of micro-tubules and actin filament occur the first step is called nucleation. In other words Lag Phase.
Reference: Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Self-Assembly and Dynamic Structure of Cytoskeletal Filaments.