The early development of the avian embryo. I. Endoderm formation

In the blastula stage the roof of the subgerminal cavity is composed of an irregular layer of cells, the nuclei of which lie in the upper or middle part of the cytoplasm. On the floor of the subgerminal cavity groups of already degenerating cells occur. They represent the vegetative pole of the blastula. Almost every cell contains glycogen, and mitotic cells show no special orientation. In the gastrula stage the cells of the area pellucida become regularly arranged as a single‐layered, cylindrical epithelium with basally situated nuclei. The yolk endoderm cells are formed from the proliferating upper layer of the area opaca. The embryonic endoderm is formed at the posterior end of the area pellucida by outgrowth of single cells from a circumscribed area, the primitive plate. This plate eventually bends inward to form a typical archenteric canal, through which endoderm continues to invaginate from the epiblast. The endoderm spreads in a cranial and lateral direction until it has formed a complete layer. The epiblast cells lose their glycogen as they invaginate to form endoderm, which is free of glycogen. In the area opaca the upper layer and the yolk endoderm contain glycogen. The mitotic cells of the epiblast of the area pellucida are always orientated horizontally, but in the primitive plate and archenteric canal they are orientated vertically as well.