Figure 1.

The appearance of germline and somatic cells in the evolution of metazoans. Two Volvocinian genera that were used by Weismann to illustrate the difference between (A) a homoplastid organism (P. morum) consisting of a single cell type and the evolution of (B) a heteroplastid (V. minor), which has evolved to consist of two primary cell types: the germline cells (Kz) that form the internal embryos and somatic cells (Sz), which are subject to death upon which the developing embryos are released.