Understanding Human Embryonic and Fetal Development

Human embryonic and fetal development is a long process (approximately 40 weeks) beginning with fertilization and ending with birth. During the first trimester women may experience morning sickness (resulting in excessive stomach acid), frequent urination (from increased fluids passing through kidneys), sleeplessness (from hormonal changes), swelling of breasts (from hormonal changes), weight changes, and food cravings (http://www.visembryo.com/baby/pregnancy1.html).

Development in organisms varies. Gestation periods for bears are up to 240 days, chickens 22 days, pigs 130 days, and squirrels 44 days (Dohtery, 2006, http://www.infoplease.com/ipa/A0004723.html ). For example, kittens are born with their eyes shut for the 19 to 14 days are deaf for about 14 days. They start to walk and begin to eat solid food when around three weeks old (Copley, 2009, cats.suite101.com/article.cfm/kitten_stages_of_development). However, standard development patterns emerge including fertilization, growth, and cellular differentiation.

Stages. The first step is fertilization. Fertilization begins the development process by the formation of a nucleus in a Zygote.  On the second day, the cells divide.  On the third day the embryo divides into four cells. On the fourth day there is an eight cell division. On the fifth day the embryo becomes a ball with many cells and is called a Morula. On the sixth day the embryo becomes a Blastocyst. Utrine fluid flows inside the blastula (interior of Morula) and surrounds the inner cells, Blastomeres, which will become the embryo. Implantation begins on the seventh day. Gastrulation, which involves major cell formation, begins the formation of tissues for skin, nervous system, internal organs, linings, muscle, bones, and heart. On the 10th day, the Blastocyst cavity becomes the primary yoke sack cavity. On the 14th day, the primary yoke sack collapses. By the fourth week, heart tubes fuse to form a heart which begins to beat. The heart is the first organ to form. Limbs and eyes begin to form on the fifth week. Oral and nasal cavities form during the sixth week. Fingers, eye lids, and genital structures start to form during the seventh week. The Fetal period begins during the eight week with the early formation of organs and the yoke sack now is part of the umbilical chord. During the 10th week, fingernails and intestines form. Limbs are well developed and toe nails form during the 15th week. Body hair, lanugo, is visible during the 20th week. The Fetus becomes viable by the 30th week.

Critical stages. Critical stages for human embryonic development include fertilization and implantation, which involves a complex messaging system involving precise steps. Any point of development embryos which stop growing can be aborted or reabsorbed. Additionally, if incorrect signals are sent, the embryo or fetus may be starved (Bear, 2001, http://www.gregbear.com/other/makeahuman.cfm).