Differentiation of the Yolk Sac of the Chick Studied by Electron Microscopy

Abstract

The chick embryo is nourished on yolk throughout its entire pre-natal life, the yolk sac being the organ responsible for the uptake of yolk and its digestion. This organ is formed from the area opaca, and during the first few days of incubation there is a striking increase in its area; in fact, the whole yolk becomes almost entirely covered by the end of the third day. This expansion is apparently the result of both proliferation and migration. It is generally considered that proliferation takes place largely in a band of tissue lying just proximal to the edge of the blastoderm (the so-called ‘syncytial’ zone or area vitellina externa). Migration has been shown to be due to the activity of highly specialized cells at the periphery (New, 1959). The purpose of Part I of this investigation was to study the relationship between the ‘syncytial’ zone and the underlying yolk, and to provide a description of the edge cells.