Early stage of development of an organism in the egg or the uterus , during which its essential form and its organs and tissues develop.
In humans, the organism is called an embryo for the first seven or eight weeks after conception, after which it is called a fetus . In mammals, the fertilized egg or zygote undergoes cleavage (cell division without cell growth) to form a hollow ball or blastocyst. During the second week following fertilization, gastrulation (cell differentiation and migration) results in the formation of three tissue types. These three types of tissue develop into different organ systems: the ectoderm develops into the skin and nervous system; the mesoderm develops into connective tissues, the circulatory system, muscles, and bones; and the endoderm develops into the lining of the digestive system, lungs, and urinary system. In humans, by about the fourth week, the head and trunk can be distinguished and the brain, spinal cord, and internal organs begin to develop. By the fifth week, limbs begin to appear and the embryo is about .33 in. (.8 cm) long. By the end of eight weeks, the embryo has grown to about 1 in. (2.5 cm) long and all subsequent change is limited primarily to growth and specialization of existing structures. Any pregnancy .