Tn any given species of vertebrates the mature red blood corpuscles, whether nucleated or non-nucleated, are characterized by a'fair degree of uniformity in size and in form. The cells are also noteworthy f o r the degree of stability of form which they exhibit, usually readily recovering the typical form after deformation by pressures or other forces.
Ordinarily, in all forms below the mammals these cells are nucleated, but several important exceptions to this generalization have been reported (Emmel, '24). Non-nucleated red blood cells have occasionally been described for Petromyzon and the chick embryo. I n the Amphibia, on the other hand, the non-nucleated erythrocytes are frequently encountered in small numbers in many forms (Triton, Rana esculenta, Salamandra maculosa, Diemyctylus torosus, Chondrotus tenebrosus, Desmognathus fusca, Anaides lugubris, Plethodon erythonotus, Spelerpes bilineatus, and Plethodon oregonensis). Moreover, in three species of Batrachoseps, at tenuatns, pacificus, and major, the non-nucleated corpuscles are found to constitute from 90 to 98 per cent of the circulating red blood corpuscles. The erythroplastids as well as the erythrocytes of these forms vary greatly in size.
I n mammals, while the erythroplastid is the type usually found in the circulation, nucleated forms may also occur under 139