Appearance and Distribution of RNA-Rich Cytoplasms in the Embryo of Xenopus laevis during Early Development

The occurrence of the dorsal yolk-free cytoplasm in the fertilized egg of Xenopus was re-examined, and the appearance and the distribution of RNA-rich cytoplasms in Xenopus embryos during early development were examined with their paraffin sections. The results show that the dorsal yolk-free cytoplasm does not occur solely in the dorsal part of the embryo but is continuous to similar cytoplasmic mass in the central and the ventral part. The whole mass of this continuous cytoplasm is denoted here as the mesoplasm. The locations of the mesoplasm in the embryo can be traced by its high RNA content during cleavage and blastulation. The cells endowed with the mesoplasm constitute a broad band about the equator of the blastula. At the lower edge of this band, the blastopore lip is formed during gastrulation. Another mass of yolk-poor and RNA-rich cytoplasm becomes distinct around every nucleus in the stage 4 embryo and is denoted here as the nucleophilic plasm. This plasm is diminished at every nuclear division and disappears in the stage 10 embryo. Origins and roles of the mesoplasm and the nucleophilic plasm were discussed and a mechanism of blastulation was suggested.