under 40%. The bulk RNA, mainly the two 

 ribosomal species, has a GC content of about 

 57%. RNA labeled through early cleavage up to 

 the blastula and all fractions except the lightest 

 one have the GC content that is markedly lower 

 than what would be expected for ribosomal 

 RNA. Remember that this is accumulation of 

 radioactivity over a period of about seven hours 

 with the radio-phosphate in the medium being 

 kept at constant specific activity, so that with 

 respect to GC content, this is very much a 

 DNA-like RNA and probably one of considerable 

 stability. 



The base composition of the light fraction 

 is highly aberrant. It is very rich in cytidylic 

 acid, and has a roughly equal distribution of 

 radio-activity among the other bases. This 

 suggests strongly that the heavy incorporation 

 of radioactivity coincident with the 4S peak 

 represents labeling of the terminal CCA se- 

 quence in transfer RNA. This is the dominant 

 synthetic process associated with RNA in the 

 course of earliest development and far out- 

 weighs the activity associated with internal 

 synthesis. What the significance of the end- 

 labeling is, I do not know, and I have not heard 

 any really useful suggestions about it. It seems 

 to be a widespread phenomenon in developing 

 systems and in other systems in which cells do 

 not grow. I should point out in this connection 

 that the embryos don't grow in any strict 

 sense. New cells are forming as a result of 

 cleavage, but there is no increase in mass. 

 Indeed, throughout the course of development 

 to the early larval stages, there is a slow but 



Table II (top) shows the base composition 

 for fractions a, B, J, &, £ as indicated in 

 Fig. 4. Remember that the DNA has a 40% GC 

 content and that these are pooled fractions from 

 heavy to light. Most of them still have a low 

 GC content except that as one approaches 

 the light end, GC content rises because there 

 is still a considerable amount of end-labeling. 

 There is clearly some ribosomal RNA accumu- 

 lating during this period. If incorporation is 

 allowed to take place from the late gastrula 

 to the prism stage, as shown by Fig. 5 (symbols 

 as for Fig. 4), which is the beginning of the 

 differentiation of definitive larval tissues, then 

 there is a predominant ribosomal RNA synthesis, 

 quite steady decline in mass, and this is be- 

 cause some carbon compounds are broken down 

 to CO 2 and water. 



If one allows radioactivity to be incor- 

 porated into RNA later, for example, with P^^ 

 as the label (Fig. 4), from late blastula to early 

 gastrula, using a long labeling period (about 

 seven hours), one gets something that looks as 

 though there were the beginning of ribosomal 

 synthesis. (Open circles, OD; closed circles, 

 counts per minute; triangles, specific activity.) 

 Notice that the specific activities are minima 

 where there are optical density maxima, sug- 

 gesting that coincidence is poor between the 

 bulk ribosomal RNA (represented by optical 

 density) and the radioactivity. The base composi- 

 tion again shows that the heterogeneous RNA 

 is still present even after a long exposure to 

 isotope. 



O.D. 



1.0- - 



0.6 -- 



0.4-- 



0.2-- 



-| r 



-1 r 



-S-i lLi; 



--2 



SP. ACT. 



3 X 10"' 



CTS/MIN. 

 -rSOOO 



-^0 



15 20 25 



FRACTION NUMBER 



30 



4000 



--3000 



I --2000 



--I000 



5 10 15 20 25 



FRACTION NUMBER 



30 



Fig. 4. 



Fig. 5. 



