WEBVTT 00:00.000 --> 00:02.233 The yolk sac contains early reproductive cells 00:02.267 --> 00:05.800 called germ cells. 00:05.833 --> 00:07.267 By 5 weeks 00:07.300 --> 00:09.167 these germ cells migrate 00:09.200 --> 00:10.933 to the reproductive organs 00:10.967 --> 00:13.367 adjacent to the kidneys.