The fine structure of germ plasm in eight‐cell embryos developing from normal and inverted eggs of Xenopus laevis

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Abstract
When X. laevis eggs developed with their animal‐vegetal axis oriented normally, the mitochondria of the germ plasm in eight‐cell embryos were frequently found to adhere to each other forming branching chains, and also the aggregates of germinal granules were physically associated with adjacent mitochondria. In embryos of the same stage developing from eggs inverted by 180°, these two components were arranged more loosely, so that the whole region of the germ plasm was less compact in appearance. In these embryos, the average number of intermitochondrial adhesions was reduced to approximately one‐third of that in the controls, while the average distance from the edge of an aggregate of germinal granules to the nearest mitochondrion was almost three times the control value. The morphology of the aggregates of germinal granules was also altered: they were much less compact, often with a large central space, and the individual granular components appeared as rings organised into chains. Absence of these associations between the two main constituents of the germ plasm in inverted eggs may correlate with the reduced numbers of primordial germ cells that enter the genital ridges, i.e., the function of the germ plasm as a determinant of primordial germ cells may depend on these associations.