Hematoxylin-eosin (H&E) and Romanowsky-dye (R&D) stained preparations of hematopoietic tissues from representative animals of the five (5) classes of Vertebrata reveal the ubiquitous presence of a uniquely eosinophilic lineage in this sub-phylum. Leukocytes displaying the unique polymorphonucleus and the eosin-binding cytoplasmic granules characteristic of eosinophils are identifiable (arrowheads) in Mammalia (Homo sapiens (human, H&E) and Mus musculus (mouse, R&D)), Aves (Columba livia (rock pigeon, H&E)), Reptilia (Pogona vitticeps (Bearded Dragon, R&D)), Amphibia (Rana pipens (leopard frog, H&E)), and Osteichthyes (Tilapia aurea (Tilapia, H&E)). Scale bar = 20μm. Reproduced from 1.