The mesoderm is proliferated by primitive streak. It is formed as two layers. In front of the primitive streak an area without mesoderm is present. It is called proamnion. After 48 hours of incubation the proamnion is also occupied by mesoderm. The mesoderm is divided into dorsal and intermediate and lateral mesoderms.
The notochordal cells arrange themselves to form a cylindrical, rod called notochordal process. It will begin at hensen's node and it slowly grows. Because of its growth the primitive streak is slowly reduced. By the end of gastrulation the primitive streak is reduced and incorporate into tail bud.
The dorsal mesoderm is located on either side of notochord. It is divided into segments. They are called somites. The first pair of somites will form after 21 hours of incubation. Afterwards, for every one hour one pair of somie will add. The 24 hours old embryo contains four pairs of somites.
The intermediate mesoderm connects the dorsal mesoderm with lateral mesoderm as a stalk. Afterwards it undergoes segmentation and give kidneys.
The lateral mesoderm extends on periphery of embryo, it is divisible into extra embryonic and embryonic mesoderms. This lateral mesoderm will split into two layers. The upper layer is called somatic mesoderm and inner layer is called splanchnic mesoderm. Ectoderm and somatic mesoderm will be called somatopleure. The splanchnic layer and endoderm will be called splanchnopleure. In between the two layers of mesoderm the space is called coelome.
Thus at the end of gastrulation specific organ
forming areas started to develop.