WEBVTT 00:00.000 --> 00:02.933 Between 3 and 4 weeks, 00:02.967 --> 00:08.633 the body plan emerges 00:08.667 --> 00:11.800 as the brain, 00:11.833 --> 00:13.867 spinal cord, 00:13.900 --> 00:16.300 and heart of the embryo 00:16.333 --> 00:17.433 are easily identified 00:17.467 --> 00:22.400 alongside the yolk sac. 00:22.433 --> 00:24.300 Rapid growth causes folding 00:24.333 --> 00:27.867 of the relatively flat embryo. 00:27.900 --> 00:29.300 This process incorporates 00:29.333 --> 00:30.867 part of the yolk sac 00:30.900 --> 00:31.967 into the lining 00:32.000 --> 00:33.967 of the digestive system 00:34.000 --> 00:35.433 and forms the chest 00:35.467 --> 00:36.467 and abdominal cavities 00:36.500 --> 00:38.633 of the developing human.