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The intraembryonic distribution of selectively stained (periodic acid-Schiff positive) primordial germ cells (PGCs) was investigated in morphologically staged chick embryos from the stage when they first arrive in the embryo proper until they have become firmly established in the gonadal anlage. The number and precise localization of the PGCs at each developmental stage have been recorded and have provided significant data on the mechanisms involved in the transport and disposition of the PGCs, particularly their colonization of the gonadal primordium. In general, the PGCs of the chick measure between 12 and 14 μ in diameter and possess abundant intracytoplasmic deposits of PAS-positive glycogen. They originate extraembryonically and are transported passively by the circulating blood to all vascularized parts of the developing embryo. The time of their first appearance within the embryo proper (stage 12) coincides with the onset of cardiac propulsion and blood circulation. Thereafter, the number of PGCs increases steadily from an average of 30 at stage 13 to an average of 894 at stage 17. In all stages examined the PGCs are evenly distributed on the right and left sides of the body. Initially, the PGCs are observed throughout the existing vascular channels, particularly in the heart, great vessels, and the small vessels of the cephalic mesenchyme. Many are found in such incongruous sites as the notochord, neural and surface ectoderm, and endoderm. By stage 15, however, the majority of PGCs are concentrated in the bilateral future gonadal region, i.e., an extensive longitudinal area of mesodermal tissue surrounding the medial portion of the intraembryonic coelom caudal to the place of exit of the omphalomesenteric arteries. The pattern of distribution within the gonadal territory varies with the developmental stage and appears to be determined by the morphological and concomitant vascular changes which occur there. At stage 15, for example, the dorsal aorta is situated directly medial to the medial angle of the gonadal territory and gives off splanchnopleuric branches which form a capillary network below the floor of the coelom. Because this is the major blood supply of the gonadal region at this stage, the vast majority (98%) of the intragonadal PGCs are confined to the capillary stroma and the epithelium of this zone. The small caliber of the capillaries compared to the large size of the PGCs serves to impede circulation here so that the PGCs become halted and then are able to begin their active amoeboid movement and to invade the overlying coelomic epithelium. Subsequent medial displacement of the coelomic cavity and aorta permits a gradual overlapping of the roof of the coelom by the dorsal aorta and concomitant alterations in the vascular relationships so that by stage 18 the majority of PGCs come to occupy the medial angle and roof zones, either by migrating from the floor region or by actively penetrating the aortic wall.