By

Studies on the nucleic acids during embryonic development revealed that there is a cytoplasmic reserve of deoxyribonucleic acid (DNA) in various inverte-brate and vertebrate eggs (Elson and Chargaff, 1952; Brachet, 1954; Schwartz, 1970; etc.). This reserve is used for the embryonic nuclei during early develop-ment. It is thought that the starting time of DNA synthesis depends on the quantity of the cytoplasmic reserve. For instance, DNA synthesis begins in the early cleavage stage of sea urchins and the cytoplasmic reserve of DNA is very small (Elson et al., 1954). In amphibians, active synthesis of DNA appears at the late blastula or early gastrula stage and the DNA reserve within the cytoplasm is very large (Chen, 1960). It could be supposed, therefore, that the amount of DNA reserve is proportional to the egg size and that DNA synthesis is initiated in the early stages of development in animals having smaller eggs than in those having larger ones (Solomon, 1957; Kong et al., 1971). As to the behavior of ribonucleic acid (RNA) during embryonic development, there is a sharp difference between sea urchin and amphibian eggs. RNA content remains constant until the pluteu