Appendix A. 
171 
While implantation is occurring, the inner cell mass is also 
undergoing changes. First, the inner cell mass separates into two 
layers, the epiblast, which is next to the amniotic cavity, and the 
hypoblast, which is next to what was the blastocyst cavity but is by 
this stage called the primary yolk sac. The epiblast thus forms the 
floor of the amniotic cavity (as the amnion forms the roof) and is 
connected with the amnion around the edges. The hypoblast is 
connected around its edges with the exocoelomic membrane or 
primary yolk sac. Thus, the supporting structures, collectively called 
the extraembryonic membranes, are outside of the body that is 
starting to develop and that will eventually be born, but during 
embryonic development the membranes are also continuous with 
that body. By the end of the second week, the hypoblast has 
developed a thickened area, called the prochordal plate, that is 
located at what will be the cranial (head) end of the individual. In 
fact, the prochordal plate shows where the mouth will develop. 
As the third week of development begins, dividing cells pile up in 
a line to form a thicker band in the epiblast. The line or band starts 
nearly directly across from the prochordal plate, and extends from 
the edge toward the center of the embryonic disc. The band is called 
the primitive streak. In many policy discussions, the appearance of 
the primitive streak is an important boundary. This summary will 
continue just a little longer, in order to discuss briefly the nature of 
the primitive streak. 
The end of the primitive streak that is toward the middle of the 
disc (nearer the prochordal plate marking the mouth) is the cranial 
end, and this end thickens more as more cells divide. This especially 
thick end is called the primitive knot (formerly called Henson’s node). 
The end of the primitive streak near the edge is the caudal (or tail- 
ward) end. As a model, think of the primitive streak as a zipper: the 
epiblast cells that made the thickness now start to migrate across 
the surface and into the zipper of the primitive streak. As the cells 
enter the primitive streak, they do a U-tum around the edge and 
continue to migrate back the way they came but underneath the 
surface, displacing the hypoblast cells. This movement results in 
three layers, all of epiblast origin: what was the epiblast on top, the 
cells that used to be part of the epiblast but are now underneath it, 
and the cells that remain in between (Figure 9). 
These three layers get new names, and they also get newly 
specified fates for their progeny. In the same order as above, they are 
the ectoderm, mesoderm, and endoderm. The completion (during the 
third week after fertilization) of forming these three layers is called 
gastrulation. The ectodermal layer gives rise to progeny fated to 
become the skin, the nervous system, and sensory structures of the 
eye, ear, and nose; mesoderm gives rise to the skeletal and muscular 
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