In part 1 of this paper ( I ) we give evidence that the P23-capsoid of 7-particles is transformed in situ i n t o the P23*-capsid of' normal phage. Using t h e polymorphism of phage T4, we have chosen polyheads as representative of P 2 3 assemblies and giant phages as representative of P23* assemblies in order t o study their surface crystals by optical filtration of micrographs. We found for polyheads a lattice constant of 1 I 2 a with the typical hexameric, ringlike capsomer and for t h e giants a lattice constant of 124 a with quite a different capsomer morphology, of t h e t y p e (h+l). From the stoichiometry of t h e proteins composing the normal capsid we conclude that the prototner is a single P23* molecule and that the minor capsidproteins must be in singular positions on t h e surface lattice o r on t h e polyhedral head (center of capsomers, vertices, o r basal part).We extrapolate the findings on the giant head to t h e normal head and give a geometric model which is consistent with 1 , I 00 molecules o f P23* per capsid.We discuss t h e part of form inheritance contrihuted by P23 and t h e other formgiving gene products and give evidence that morphologic characters are t h e result of pairs of a reaction chain of interacting gene products. The example we give is the giant head produced by a t s mutant in gene 2 4 at 3h'C.

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