By 4 weeks the clear amnion surrounds the embryo in a fluid-filled sac.[55
] This sterile liquid, called amniotic
, provides the embryo with protection from injury.
The heart typically beats about 113 times per minute.[57
] The heart changes color as blood enters and leaves its chambers with each beat.
The heart will beat approximately 54 million (5.4×107) times before birth and over 3.2 billion (3.2×109) times over the course of an 80-year lifespan.[58
Rapid brain growth is evidenced by the changing appearance of the forebrain, midbrain, and hindbrain.
Upper and lower limb development begins with the appearance of the limb buds by 4 weeks.[59
The skin is transparent at this point because it is only one cell thick.
As the skin thickens, it will lose this transparency, which means that we will only be able to watch internal organs develop for about another month.[60
Between 4 and 5 weeks, the brain continues its rapid growth and divides into five distinct sections.[61
The head comprises about one-third of the embryo's total size.[62
] gradually becoming the largest parts of the brain.[64
Functions eventually controlled by the cerebral hemispheres include thought, learning, memory, speech, vision, hearing, voluntary movement, and problem-solving.[65