Fourth Week to Eight Week - Embryonic Development
The common changes in body shape and plan of the embryo from fourth week to eighth week are displayed in Figure. The fourth week embryo is cylindrical and has a blunt head with an extremely primitive brain. Vague rounded elevations on the lateral surfaces of the brain indicate the eye rudiments. The simple tube like heart that arose in the third week is functional, pumping blood by the umblical arteries to the placenta. Heartbeats, though, cannot be recorded yet. Oral and anal openings appear but are nonfunctional. At the end of fourth week the embryo is 1/4 of an inch long. A characteristic feature of the fourth week is the alternating series of elevated ridges and depressions, the pharyngeal pouches and grooves correspondingly. These 4 pharyngeal pouches consider to the gill arches and grooves to the gill slits of fishes.
Figure: Human embryonic development from fourth to eighth week
Though, these grooves in humans never become functional or perforated. The pharyngeal pouches in humans make the eustachian tubes (1st pair of pouches), walls of tonsils (2nd pair of pouches), thymus and parathyroids (3rd and 4th pair of pouch). Throughout the fifth week the embryo turns curled so that the head almost touches the tail. The rudimentary arms and legs (limb buds) appear about the middle of the fifth week. The head is now much larger than the trunk region. The brain is the most famous feature of the embryo. Nerves spread out in the body, paired gonads form, though not familiar yet, as testes or ovaries. In the sixth week the head is disproportionately large and the stomach bulges out due to the large liver. Future fingers are seen as indentations on the paddle shaped hands and feet. The hands develop faster as compared to the feet. This is an expression of the general rule that anterior structures grow much more fast than posterior structures. Rapid growth occurs in the facial region. By the end of the sixth week, the main systems, nervous, excretory, reproductive, muscular, circulatory, digestive and skeletal have been initiated. After the sixth week the embryo begins looking more human and all the internal organs are formed by the seventh week. With the development of brain the head achieves its normal relationship along with the body as the neck appears. The nervous system is developed adequate to permit reflex actions such as the 'startle response' to touch. By the end of eight week of development, the embryo is generally 30 mm in length and weighs less than 5 mg.