Somites form one after the other down the length of the embryo from the head to the tail, with each new somite forming on the caudal (tail) side of the previous one. The timing of the interval is not universal. Different species have different interval timing. In the chick embryo somites are formed every 90 minutes.

In which week of embryonic period most of the somites appear?

In the fourth week the embryo has a length of 1.5 to 3.5 mm. At the beginning of the week it is almost straight. On the dorsal side, one recognizes the contours of the first 4 to 12 somites (although the ectoblast lies above the mesoblast, the somites stand out from it).

What occurs during Somitogenesis?

Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelium, and dermis.