Figure 1. Preimplantation development in the mouse. Cell divisions are indicated above the horizontal arrows. The zona pellucida is in grey, and cleaving blastomeres are white. (a–d) Early cleavage stages. (e) At the 16-cell stage, compaction is underway; the outer blastomeres are shown in blue (future trophectoderm, TE), and the inner blastomeres (future inner cell mass, ICM) are in white. (f) Formation of the blastocyst. Note that the inner cell mass is completely enveloped by trophectoderm. b, blastocoel cavity. (g) By 4.5 dpc, the blastocyst has hatched out of its zona pellucida, the inner cell mass has formed primitive endoderm (not shown) which is differentiating into visceral (VE) and parietal (PE) endoderms.

Figure 2. Postimplantation mouse development (approximately 4.5–8.5 dpc). Three-dimensional schematic drawing of mouse development from implantation through early neurulation stages. The dotted line indicates the separation between extraembryonic tissues (upper portion) and embryonic tissues (bottom portion). At the time of implantation (a), the conceptus is composed of three distinct tissue lineages: trophectoderm (grey), primitive endoderm (yellow) and epiblast (blue). (b) Primitive endoderm has spread onto the mural trophectoderm where it becomes ‘parietal endoderm’ while the primitive endoderm remaining in contact with primitive ectoderm is called visceral endoderm. The polar trophectoderm of (a) has proliferated and pushed the primitive ectoderm (also ‘epiblast’) and its surrounding visceral endoderm into the blastocoel. The conceptus is now referred to as an egg cylinder. In the extraembryonic region, extraembryonic ectoderm is covered by ‘extraembryonic visceral endoderm’ while the embryonic portion of the egg cylinder is invested with ‘embryonic visceral endoderm’. Embryonic visceral endoderm will be replaced by definitive endoderm, and extraembryonic visceral endoderm forms part of the yolk sac. (c) The proamniotic cavity has expanded and, for the sake of clarity, the parietal endoderm and associated trophoblast have been removed. (d) The onset of gastrulation is indicated by the appearance of the primitive streak and mesoderm (red), both extraembryonic and embryonic mesoderm. (e) By approximately 7.0 dpc, the exocoelomic cavity is forming by coalescence of extraembryonic mesodermal lacunae, which results in the separation of extraembryonic ectoderm (future chorion) from primitive ectoderm, the upper part of which will form the future ectodermal component of the amnion. In the embryo, embryonic mesoderm is spreading anteriorly while the primitive streak is lengthening distally. (f) By about 7.5 dpc, the primitive streak has reached the distal tip of the egg cylinder, where it has condensed into the node (black). Anteriorly, the node is elongating and forming the head process (brown). Embryonic visceral endoderm is being replaced by definitive endoderm (yellow), and the embryo is preparing for cardiovascular and neural development. Posteriorly, the allantois has formed and is growing into the exocoelomic cavity, whose formation is now complete and comprised of the chorion (roof), the yolk sac (walls) and amnion (floor). The primordial germ cells (not shown) are forming near the base of the allantois. (g) By 8.5 dpc, the embryo contains five somite pairs, one pair on either side of the notochord (former head process), and the formation of which begins anteriorly. Anteriorly, the headfolds are becoming divided into three major regions of the future brain (purple), and the nascent heart and foregut are present. As the hindgut forms in the posterior region, the primordial germ cells will migrate along its dorsal mesentery to the gonads which have not yet formed. For more information, see figure 17 and accompanying legend in Hogan et al. (1994), pp. 58–59. Reproduced with permission from Hogan et al. (1994). Copyright © 1994 Cold Spring Harbor Laboratory Press.