Comments on a new model by two physicists of the assembly of microtubules from the monomer tubulin. Discussion of the dynamics of the growth and, conversely, the shrinkage of microtubules in `Physical Review Letters,' 3/8/93; Study by Tim Mitchison and Marc Kirschner in 1984; Argument from Marileen Dogterom and Stanislas Leibler; Details of the study.