These cells correspond in size with the macrocytes, varying from 9 µ to 20 µ. The cell is usually circular or oval, but may show marked irregularities. The protoplasm appears swollen and shows marked polychromato-philia, is varying in color from yellow to purple. The nucleus is large, varying from 6 µ to 10 µ and may be situated either centrally or peripherally. It varies greatly in shape and in staining, frequently being poorly defined and showing very faint basic staining. Sometimes there may be present mitoses or various stages of karyorrhexis with fragmentation, vacuola-tion and minute subdivisions of chromatin. Their presence in the blood probably indicates a reversion to an embryonal type of blood formation. They are evidences of a degenerative process of the blood-forming organs. When present as a majority of the nucleated red cells, particularly if the cells are unusually large and the nuclei show unequal mitoses, a diagnosis of primary pernicious anemia can be made.

Microblasts are nucleated red cells of from 2 µ to 5 µ, are not frequently found and do not appear to have any special significance.

Number of red cells may vary greatly and such variations are not uncommon. Normally there is found in a cubic millimeter of blood five million cells in the male and four and a half million in the female. The number may be lowered temporarily in women as a result of menstruation, child-birth and lactation. It also varies somewhat according to the age, being particularly high at birth and a few days after, but soon becomes normal and remains with but little change during health. There is also some increase in number at high altitudes.

Oligocythemia refers to a decrease in the number and is usually associated with a decrease in the total amount of hemoglobin. It may be temporary or permanent, according to the underlying cause, such as acute hemorrhages or in cases of chronic pernicious anemia. In the latter a count as low as 100,000 red cells has been reported.

Polycythemia refers to an increase in the number of red cells. Conditions that cause a great loss of liquid from the body, as in profuse diarrheas, may cause a greater concentration of the blood and an apparent increase in the cells. It may occur in phosphorous poisoning, acute yellow atrophy of the liver, and in some cases of general hepatic insufficiency.