Fig. 1. A. Drawing of a 17-day-old embryo in gastrulation stage, dorsal view, with the amnion removed. B. Cross-section of a 17-day-old embryo through the primitive streak. The primitive streak represents invagination of epiblast cells between the epiblast and hypoblast layers. Note that the epiblast cells filling the middle area form the mesodermal layer. C. Cross-section of the embryo at the end of the third week showing the three definitive germ layers: ectoderm, mesoderm, and endoderm. (Cook CS, Sulik KK, Wright KW: Embryology. In Wright KW [ed]: Pediatric Ophthalmology and Strabismus, pp 3–43. St Louis: Mosby, 1995.