(1) The spleen arises in a thickening of the mesenchyme of the foregut, just after that mesenchyme has become free from yolk granules.
(2) It is, at first, a mass of mesenchyme cells, round about which are comparatively large venous sinuses without any endothelial walls; later the cells become arranged to form trabeculæ across these sinuses, which thus get broken up into the channels of a spongework.
(3) The afferent and efferent veins are in very close connection with the veins from the intestine and to the liver respectively. The arterial supply of blood develops from the cœliac artery rather later.
(4) The organ remains throughout ontogeny embedded in the sheath of the foregut, and is therefore inconspicuous.