Representative images of chicken embryos used for RNA preparation and subsequent CAGE sequencing. Top panels show embryos in the first day of embryonic development (21 days in total), equivalent to the first five weeks of human gestation (first three weeks of human post-fertilization development). Bottom panels show embryos from the establishment of circulatory system to about a week before hatching, similar to the beginning of third trimester in human development.
[Image modified from the original (Figure 1A) in Lizio, M. et. al. (2017). Systematic analysis of transcription start sites in avian development. PLOS Biology, 15(9), e2002887. doi:10.1371/journal.pbio.2002887 under the CC by 4.0 license.]