In the observations carried on microstructures of the erythrocyte series at the embryonic stage in the chick blastoderm by a phase-contrast microscope the author obtained such peculiar findings on the various cells in motion that have hitherto been unobservable by usual stainings and supravital staining. First of all, the primitive erythroblasts (Megaloblast, Sabin) have been found to possess giant nucleus with thin nuclear membrane, reticulum consisted of extremely fine fibers, nucleoles situated approximately in the center and of irregular round shape to be the darkest spot in the nucleus with a coarse network-like structure on the surface, and numerous short rod-like mitochondria arranged in parallel with the nuclear membrane. Secondly, the primitive proerythrocyted (erythroblast stage I, Sugiyama) have the nucleus with coarse but thick reticulum and with round or irregularly round small nucleoles showing deep or light colored spots on the surface. Mitochondria are arranged like a band and are variously shaped as J, U, V, or S, and most of them are situated around the nucleus and a portion of them in the outer layer. Finally, primitive erythrocytes (erythroblast stage II, Sugiyama) have the nucleus with thick axle-like reticulum but nucleoles are obliterated, showing no mitochondria.