The development of the circulating blood volume of the chick embryo.

The circulating blood volume of the chick embryo was determined from the 4th up to the 18th day of hatching. In contrast to former studies, there was employed a radioisotope dilution method with albumin-bound I131. The findings are in close correspondence to those of the earlier studies. The blood volume does not display an entirely perfect curve of exponential growth, i.e., the doubling time increases steadily. The blood volume attains a peak value between the 16th and 18th day and decreases somewhat toward the end of the hatching period. There has been postulated a reduction of total red cell volume and hemoglobin caused by the involution of the extraembryonic circulatory system. The destruction of the erythrocytes seems to take place in the endodermal epithelium of the proximal yolk sac, where an accumulation of iron could be demonstrated on the 19th and 20th days.