Fourth week (stages 8 to 10; ca. 23-28 days)
During the fourth week (stages 8-10) the first morphologic characteristics that typify vertebrates are formed. Thus the embryo, flat at the beginning, comes to have a shape that is typical for vertebrates. This is primarily due to the folding of the germinal disk, to the differentiation of the somites and to the rapid development of the nervous system.
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).
At the same time, out of the neural plate, the process of neurulation produces a hollow cylinder whose rostral and caudal ends still stand wide open.
At around the 28th day (stage 10), at the same time as the formation of the abdominal wall and the development of the somites progresses, the first two pharyngeal arches appear.
The first arch is the mandibular arch. It divides itself into a maxillar and a mandibular process.
The second arch is the hyoid arch.
The heart is visible as a medial prominence and begins to beat at around the 21rst day.
Cervical and caudal flexures have the effect that the embryo takes on its characteristic C shape.