ABSTRACT
At first, the disc is bilaminar, made up of a layer of columnar cells (embryonic ectoderm) lying on a layer of flattened cells (embryonic endoderm). At 16 days, the primitive streak appears in the median plane of the embryonic disc. This event is the equivalent of verte brate gastrulation in lower forms, whereby an interme diate layer of mesoderm is formed between the outer ectoderm of the larval body and the inner endoderm layer lining the gut tube. In humans, proliferating cells pass through the primitive streak, forming a trilaminar disc with an intermediate layer of embryonic mesoderm. Comparative evidence (Sze 1953) suggests that the growth rate of the human disc falls abruptly in the early 3rd week (Figure 3). From then into the 4th week, the trilaminar disc rolls up on itself (flexion), so that the endodermal layer forms the lining of the gut at 24 days (Figure 4a).
