THE MIGRATION OF THE PRIMARY SEX-CELLS OF FUNDULUS HETEROCLITUS

Abstract

1. The earliest primary sex-cells found in Fundulus were located in the peripheral endoderm, lateral to the posterior half of the 46-hour embryo. No sex-cells were observed in that part of the embryo which develops from the head fold.2. The germinal path leads from the peripheral endoderm, into the border of the undifferentiated endodermal cell mass. When this cell mass splits to form gut endoderm and lateral mesoderm, the sex-cells proceed medially with either layer. By the time the gut is formed, these cells are lateral to it; they all eventually become located in the splanchnic mesoderm of this region. From here the sex-cells migrate dorsal to the hind gut, thence to the region ventral to the Wolffian ducts. Here they become surrounded by peritoneal cells which form the somatic portion of the gonads. From this position the germ gland anlagen are shifted back to their final location dorsal to the gut.3. There is very little multiplication of the sex-cells during the period of migration. Division apparently takes place in the extra-embryonic area, and is not renewed to any marked extent until after the sex-cells become located in the germ glands.4. The constant distinguishing characteristics insure positive identification of these cells throughout all phases of their migration, and leave no reason to question their identity as being the "primordial germ cells" of previous writers.5. Migration is passive, being due to forces of growth which are altogether external to the cells themselves. These forces of growth are factors common to the development of the organs formed in the body of the teleost embryo.6. Evidence derived from this study of Fundulus is an absolute harmony with the theory of early segregation of these primary sex-cells.