ABSTRACT
Megaloblastic cytology is not only readily recognizable but is also widely appreciated. The yolk sac-derived primitive generation nucleated erythrocytes of man display obvious megaloblastic features and the cells have been termed megaloblasts by some investigators. Even in man the primitive yolk sac-derived erythrocyte is followed by a transient hepatic generation that generate fetal hemoglobin rather than the adult version. The origin of the hemopoietic stem cell for post-primitive generation(s) erythrocytes that develop in the yolk sac, however, is another important topic and is discussed later in the text. Blood forming activity of the yolk sac is intensified on the chick’s fourth day of incubation. The mature primitive generation erythrocyte is the dominant form in circulation at 125–140 hr. The typical maximal life span of chick primary generation erythrocytes is in the range of 22 days. The primitive generation erythrocytes that are produced by a given species are distinctly different from the permanent, definitive version that the animal subsequently elaborates.
