Avian Spare Yolk and Its Assimilation

Abstract

IT is well known that at the time of hatching a bird still retains a portion of unassimilatcd yolk material. About two days before the embryo is ready to bc hatched, a rapid growth and unfolding of the tissues at the umbilicus cause that portion of the yolk which has as yet not bccn absorbed to bc enclosed within the abdominal cavity. After enclosure of the yolk, its utilization goes on as before until, under normal conditions, it has bccn entirely absorbed. Since the yolk sac is actually a divcrticulum of the small intestine, its material is absorbed directly from the sac into the blood stream. The spare yolk, in this manner, is able to supply the developing chick with a store of nutriment which enables it to survive for a limited period after hatching without an additional supply of food. The rate of assimilation of the spare yolk has bccn studied primarily in the chick (Virchow, 1891; Iljin, 1917; Schilling and Blccckcr, 1928; Parker, 1929; Jull and Heywang, 1930; Romanoff and Romanoff, 1933; Romanoff, 1943; and Fronda, Bancz, and Moncgas, 1937). Very little is known about the absorption of yolk in other species of birds as well as about changes in its chemical composition following hatching. For this reason the work was extended to include several species of birds and chemical changes in the yolk which, it was thought, might indicate preferential absorption of certain food material. EXPERIMENTAL Amounts o[ spare yolk in various species.-In the newly hatched chick the amount of unassimilated yolk material has been variously determined by Iljin (1917) at hatching as 8.19 gins., by Virchow (1891) at 12 hours as 5.34 gms., and by Parker (1929) and Jull and Hey-wang (1930) at 24 hours as 4.41 gms. and 5.86 gms., respectively. In observations by the author the average yolk-sac weight immediately after hatching was 6.16 gins. in Leghorn and 7.25 gms. in Brahma chicks. The amount of spare yolk found at hatching in various species of birds is given in Table 1. The weight of the yolk sac varied closely with the weight of the bird. It also varied consistently with the original egg weight; the larger the egg, the heavier the yolk sac. On the other hand, the percentage of yolk weight to bird weight did not show such consistent variation. In general the smaller birds had a lower percentage of yolk weight than the larger birds, the values extending from ten to nearly twenty-five per cent.