J. R. SHAVER 273 



It is apparent that there is no discernible gradient in the dis- 

 tribution of ribonucleoprotein in the development of sea urchins 

 so far studied. Brachet (1950) using toluidine blue and ribonu- 

 clease to indicate the localizations of these substances, followed 

 the distribution of both deoxyribonucleic acid and ribonucleic 

 acid in sea urchin embryos. Although there was an overall de- 

 crease in cytoplasmic basophilia, interpreted as a decrease in 

 ribonucleic acid, there was no evidence of a giadient-like distri- 

 bution of this substance. The micromeres were especially rich in 

 ribonucleic acid, but, as Brachet states (1950, p. 451): ". . . we 

 are still simply dealing with a local accumulation of the ribonu- 

 cleoproteins, which do not show a true gradient visible under the 

 microscope. Of course, one cannot a priori reject the idea that 

 particular ribonucleoproteins bound to granules, for example, are 

 distributed in gradient fashion, but this supposition is gratuitous 

 at present." 



Shaver ( unpublished results ) has obtained essentially the same 

 picture of the ribonucleic acid distribution in developmental 

 stages of two sea urchin species. Toluidine blue or methyl green 

 pyronine were used after fixation of whole embryos on coverslips 

 with Zenker's fluid ( without acetic acid ) or Altman's fluid. There 

 did not appear to be any evidence of a gradient of dye absorption 

 in any of the stages observed. 



Cytochemical and biochemical evidence has been increasing 

 concerning the association of ribonucleic acid with protein syn- 

 thesis in the cell (Brachet, 1952; Pollister, 1954). The fact that a 

 larger part of the ribonucleic acid of adult cells is associated with 

 the microsome fraction (cf. Lindberg and Ernster, 1954), empha- 

 sizes the importance of these particles in protein synthesis. There 

 seems relatively little information, however, about the concentra- 

 tions of ribonucleic acid in the particulate fractions of sea urchin 

 embryonic cells. If the assumption is made that ribonucleic acid 

 is progressively bound to microsomes in the cells of the sea urchin 

 embryo, there appears to be no gradient in the distribution of 

 these particles. 



Unlike microsomes, mitochondria stain relatively weakly with 

 basophilic dyes ( see, however, Lindberg and Ernster, 1954, for a 



