1. The primordial germ-cells of Sphenodon originate in the yolk-sac endoderm of the area opaca all round the embiyo, but chiefly in a crescentic area in front of it.
2. They differentiate first at a very early stage of development before the differentiation of the medullary plate.
3. The primordial germ-cells are characterized by their very large size, in comparison with all the other embryonal cells, and by their content of small yolk-spherules which are sub-equal in size.
4. The primordial germ-cells migrate through the yolk-sac endoderm and mesoderm, apparently by their own power of amoeboid movement. Many of them enter the blood-islands and the sinus terminalis.
5. The primordial germ-cells enter the embryo either (1) passively in the venous blood-stream, or (2) actively by migration through the extra-embryonal endoderm and splanchnic mesoderm into the lateral walls and the mesentery of the midgut groove.
6. The primordial germ-cells in the circulation reach the neighbourhood of the germinal ridges in the dorsal aorta or its branches. They then penetrate the walls of the vessels and migrate through the intervening tissues, together with those that have reached the base of the mid-gut mesentery by way of the splanchnic mesoderm and the mid-gut wall, to the germinal ridges.
7. Many primordial germ-cells get lost during their migration. This is especially true of those travelling in the blood-stream. Such aberrant primordial germ-cells are found occasionally in almost any part of the embryo,, but occur most often in the head, especially in the region of the fore-brain. They ultimately disappear.
8. The primordial germ-cells enter the forming germinal ridges before the coelomic epithelium covering them has begun to proliferate.
9. The primordial germ-cells, having reached the germinal ridges, lose their characteristic yolk-content and enter on the prophase of the heterotypic division.