Expression is seen in a patch of cells in the anterior portion of the mesodermal primordium.
These cells invaginate with the ventral furrow anterior to the cephalic furrow and become located laterally to the stomodeum which in part also expresses srp . Slightly later these cells differentiate into prohemocytes which migrate into the head. Subsequently, these cells become distributed throughout the body and differentiate as mature hemocytes, at which point srp expression is downregulated. During germ band extension , the only expression seen is in the cephalic mesoderm. Before germ band retraction, serpent -expressing mesodermal cells appear in segmentally repeated structures reminiscent of the mesodermal expression of nautilus.