Читать книгу Lifespan Development - Tara L. Kuther - Страница 152

By the third week after conception, the developing organism—now called an embryo—begins a period of structural development during which the most rapid developments of the prenatal period take place. All of the organs and major body systems form during this embryonic period. The mass of cells composing the embryonic disk develops into layers, which will develop into all of the major organs of the body. The ectoderm, the upper layer, will become skin, nails, hair, teeth, sensory organs, and the nervous system. The endoderm, the lower layer, will become the digestive system, liver, lungs, pancreas, salivary glands, and respiratory system. The middle layer, the mesoderm, forms later and will become muscles, skeleton, circulatory system, and internal organs.

During the third week, at about 22 days after conception, the endoderm folds to form the neural tube, which will develop into the central nervous system (brain and spinal cord) (Webster et al., 2018). Now the head can be distinguished. A blood vessel that will become the heart begins to pulse and blood begins to circulate throughout the body. During days 26 and 27, arm buds appear, followed by leg buds on days 28 through 30. At about this time, a tail-like appendage extends from the spine, disappearing at about 55 days after conception (Sadler, 2015). The brain develops rapidly and the head grows faster than the other parts of the body during the fifth week of development. The eyes, ears, nose, and mouth begin to form during the sixth week. Upper arms, forearms, palms, legs, and feet appear. The embryo shows reflex responses to touch.

During the seventh week, webbed fingers and toes are apparent; they separate completely by the end of the eighth week. A ridge called the indifferent gonad appears; it will develop into the male or female genitals, depending on the fetus’s sex chromosomes (K. L. Moore & Persaud, 2016). The Y chromosome of the male embryo instructs it to secrete testosterone, causing the indifferent gonad to create testes. In female embryos, no testosterone is released, and the indifferent gonad produces ovaries. The sex organs take several weeks to develop. The external genital organs are not apparent until about 12 weeks.

At the end of the embryonic period, 8 weeks after conception, the embryo weighs about one seventh of an ounce and is 1 inch long. All of the basic organs and body parts have formed in a very rudimentary way. The embryo displays spontaneous reflexive movements, but it is still too small for the movements to be felt by the mother (Hepper, 2015). Serious defects that emerge during the embryonic period often cause a miscarriage, or spontaneous abortion (loss of the fetus); indeed, most miscarriages are the result of chromosomal abnormalities. The most severely defective organisms do not survive beyond the first trimester, or third month of pregnancy. It is estimated that up to 45% of all conceptions abort spontaneously, and most occur before the pregnancy is detected (Bienstock, Fox, & Wallach, 2015).