Figure 1.

Insertion of a new module of development distinguishes early mammalian embryogenesis from that of its predecessors. Most animals deposit very large eggs in the external environment where they develop into a feeding organism without nutrition or growth. Cleavage cell cycles divide the egg into blastomeres that immediately initiate pattern formation and differentiation to produce an embryo, called phylotypic, because all of the species of a phylum look similar at this stage. Even the embryos of other phyla are similar in shape and size at this stage. Mammalian embryogenesis, as exemplified by the mouse, begins with a much smaller egg (magnified somewhat here) and a distinctly different early program of development. The cells resulting from the initial divisions do not gastrulate. First, extra embryonic tissues are produced as the embryo grows enormously and rapidly. Only then does it gastrulate and establish strong parallels to the ancestral program of embryogenesis. The mammalian module of development (boxed) is inserted early during development (*) so that the earliest divisions are dramatically changed and many of the attributes of the early events of the ancestral program are deferred along with gastrulation.