THE mode of origin of blood in Teleosts is considered to represent an exception in the development of Vertebrates. Since the observations of Swaen and Brachet1 the intermediate cell mass of Oellacher is considered to be the earliest primordium of blood and blood-vessels. From its primitive situation between the somites and the lateral plate this mass will be pushed by the ventral mesoderm into the space which is limited on the dorsal side by the chorda and scleromyotomes, and by the entoderm on the ventral side.