In embryonic period, all the following are true EXCEPT
Table 8-1 Milestones of Prenatal Development WK DEVELOPMENTAL EVENTS 1 Feilization and implantation; beginning of embryonic period 2 Endoderm and ectoderm appear (bilaminar embryo) 3 First missed menstrual period; mesoderm appears (trilaminar embryo); somites begin to form 4 Neural folds fuse; folding of embryo into human-like shape; arm and leg buds appear; crown-rump length 4-5 mm 5 Lens placodes, primitive mouth, digital rays on hands 6 Primitive nose, philtrum, primary palate 7 Eyelids begin; crown-rump length 2 cm 8 Ovaries and testes distinguishable 9 Fetal period begins; crown-rump length 5 cm; weight 8 g 12 External genitals distinguishable 20 Usual lower limit of bility; weight 460 g; length 19 cm 25 Third trimester begins; weight 900 g; length 24 cm 28 Eyes open; fetus turns head down; weight 1,000-1,300 g 38 Term
formation of blastocyst by 8 days
formation of ectoderm and endoderm by 2 weeks
formation of mesoderm by 6 weeks
formation of human like embryo by 4 weeks
General anatomy