The monocytes of lampreys [Lethenteron camtschaticum (adult), L. reissneri (adult), L. hattai (adult, ammocoetes)] were round or oval, low nucleus/cytoplasm ratio, and have striated (two-layered) granules (MoG). The MoG consisted of a basophilic inner layer (L0) and a chromophobic outer layer (L1). The L0 were positive for acid phosphatase, α-naphthyl acetate esterase, naphthol AS-D chloroacetate esterase and Sudan black B. Also, the L0 showed orthochromatic (blue) with toluidine blue. However, ꞵ-glucuronidase, α-naphthyl butyrate esterase and peroxidase were not detected in the monocytes, which were negative for periodic acid Schiff reaction, alcian blue, Sudan III, and oil red O. Kenji Kiyono reported the two types of monocytes (as blood histocytes), real and dubious monocytes, from the blood of hagfish Eptatretus burgeri. He observed both monocyte types in grown-up hagfish, but only real monocytes in undeveloped (details not stated) hagfish. His real monocytes in undeveloped hagfish were speculated as the monocytes of lamprey (mistaking the specimen), and the real monocytes in grown-up hagfish as poorly stained neutrophils. The dubious monocytes of Kiyono were considered as the real monocytes of hagfish.

Artur Pappenheim reported a poor description of the morphology of blood neutrophils from lamprey (Lampetra planeri； adult and ammocoetes) and hagfish (Myxine glutinosa) in Folia Haematologica (volume 8, 1909). Here, we inferred on the morphological characteristics of cyclostome neutrophils observed by Pappenheim based on his descriptions and our previous reports. He recognized neutrophilic leukocytes (mature neutrophils) in lamprey (adult and ammocoetes) and hagfish, and neutrophilic myelocytes (immature neutrophils) in ammocoetes and hagfish. This means the existence of specific granules and azure granules. However, the specific granules were considered as an inner layer (L0) of the striated granule (NG2), and the azure granules outer (L1) layer of the striated granule (NG2). The specific granules (=L0 of NG2) would probably have been stained with triacid (purple), acid dyes, and May-Grünwald·Giemsa (MGG； purple), but not with methylgreen-pyronine (MP). The azure granules (=L1 of NG1a) would also have stained purple with MGG, but negative with triacid, acidic dyes and MP.