FIG. 3.

Development of TR of ungulate species from mural trophectoderm. A) Pig conceptus at about Day 13 of pregnancy, at which stage each can achieve a length of 1 m or greater. The position of the relatively undeveloped embryonic disc is arrowed. B) The formation of the embryonic plate in the goat conceptus, illustrating the loss of polar TE during the early stages of conceptus elongation and the growth of mural TE and underlying extraembryonic endoderm. The expanded blastocyst (B1) is at around Day 10 of pregnancy, but within a few days polar trophectoderm disappears (B2), and the embryonic plate is formed (B3). The location of the TSC “niche,” if it exists in such conceptuses, is unknown. The diagrams are redrawn (with permission) from John R. Fuller's illustrations in Austin [149].