Phosphorylation of brain and liver ribosomal proteins during early development of chick embryo

32P-ortophosphate was introduced intraamnionally into 5, 9, 12, 15 and 19 day-old chick embryos. After 4 hours the specific activity of brain and liver ribosomal proteins and their fractions were determined. It was found that the specific activities of those in both tissues were highest at the early stages of development, and they declined rapidly to reach the lowest value at day 19. The observed differences between brain and liver ribosomal proteins are consistent with an unequal rhythm of ontogenesis.