The embryonic time comprises 56 days, i.e., 8 weeks from the moment of fertilization. This time span is divided into 23 Carnegie stages and the stage classification is based solely on morphologic features. Carnegie stages are thus neither directly dependent on the chronological age nor on the size of the embryo. This can be illustrated by two examples: The closure of the rostral neuropore occurs by definition in stage 11 and that of the caudal neuropore in stage 12. Further, between the 25th and 32nd days of the pregnancy, the stages are determined according to the number of the somites (stages 9-13) that have been engendered. The individual stages thus differ in how long they last.
During the embryonic period most of the organ systems are established and this with an enormous rapidity. Cell divisions, movement and differentiation are the basic processes taking place during this phase. It is thus hardly surprising that this pregnancy phase is very vulnerable and that deformities are produced most often during this time. The type of deformity depends on the embryonic developmental stage.