Identification

• The word "embryo" comes from the Greek word "embryon," meaning "young animal." The embryonic stage begins around three weeks after fertilization. During this period, the embryo will develop most of its body organs and external definition. By eight weeks after fertilization, or the 10th week of pregnancy, the embryonic phase is complete, and the fetal period begins.
  
  

Significance

• The embryonic period is a significant time. Most birth defects occur during the first eight weeks of pregnancy because of the body organs' lack of development. It is crucial that pregnant mothers abstain from drugs and alcohol during this time. The primary development of most of the major organs (i.e., heart, lungs) occurs during this time.
  
  

The Carnegie Stages: Identification

• The University of Michigan's informative embryology website, titled "The Multi-Dimensional Human Embryo," provides illustrations and descriptions of the Carnegie stages of embryonic development. These stages, named after the Carnegie Institute where they were developed in the early 1900s, refer to the external appearance of the embryo, rather than the embryo's age. Because it is difficult to pin down the exact date of conception, doctors use these stages and their corresponding physical characteristics to identify the age of the embryo.
  
  

The Carnegie Stages: Time Frame

• There are 23 stages in total. During the first 13 stages, remarkable developments occur. At stage one, the embryo is 0.1 to 0.15mm in size. During the next 15 days, the embryo will grow to 0.4 mm, and a new cell layer, called the ectoderm, will begin to form. In the next three days, the neural plate will begin to develop, forming the basis for the nervous system. By the fifth week of pregnancy, the embryo is visibly human.
  In the next 10 stages, the embryo will begin to develop limbs, and the spine begins to straighten. The upper lip will be completely formed, and the nasal passages are developed. Eyelids and nostrils are distinguishable and the major organs have completed the first stages of development.
  
  

Potential

• Health problems that occur in the bodies of adults sometimes begin in the embryonic stage of development. Embryonic stem cell research has made headway in determining these links, although its practices are sometimes considered morally unacceptable. Infertility, for example, might be rooted in cellular defects that occur in the embryonic period. In a study published in the October 2009 issue of Nature, conducted by researchers from Stanford University's School of Medicine, scientists were able to reproduce human germ cells by manipulating embryonic stem cells in the laboratory. This new accomplishment provides fresh insights into the link between adult disease and embryonic stages of development.