The Conversion of Yolk into Cytoplasm in the Chick Blastoderm as shown by Electron Microscopy

Three main types of yolk drop are present in the area pellucida of the chick blastoderm. They have been given the names ‘complex yolk drop’, ‘type A yolk drop’, and ‘type B yolk drop’. Together they have been called secondary yolk to distinguish them from the extra-embryonic or primary yolk. It is concluded that the complex yolk drops are early stages in the conversion of yolk. They contain granular material which resembles that found in the primary yolk. Type A drops are present in the complex yolk drops and appear to be formed from the granular material. Type B drops, mitochondria, vacuoles, and membranes are also found within the complex drops. The membranes around the complex drops appear to break down, releasing the contents of the yolk drops into the cell. Most type A drops appear to become converted into a number of circular, or possibly tubular, bodies. These are believed to be predominantly lipoproteins. It is concluded that the membrane around each type A drop breaks down and these circular bodies are then released into the cytoplasm where they may contribute to the endoplasmic reticulum. Other type A drops are composed of numbers of micro-particles. They closely resemble the cytoplasmic micro-particles and may become part of them. Evidence is presented for considering these micro-particles to be precursors of ribonucleoproteins. The type B drops persist in the cytoplasm after the complex drops and the type A drops have almost completely disappeared. Evidence is presented for considering them to be fatty drops. Primary yolk from the hen’s egg has also been examined.