200 NUCLEIC ACIDS AND GROWTH 3 



absolute specificity of the methods used: the larger the egg, the more difficult 

 becomes the analytical problem of estimating very small amounts of DNA in the 

 presence of enormous quantities of foreign substances. In our own experience 

 (Brachet, 1954!)), eggs of frogs contain small amounts of substances which give 

 colour reactions for deoxypentose and yield small quantities of thymine on 

 hydrolysis: but these substances appear to be bound to the yolk platelets, which 

 become digested at a late stage of development only: it is difficult to visualize how 

 they could be utilized as DNA precursors during early cleavage. In the egg of the 

 hen, it is also somewhat perplexing to find such a large store of DNA, when we 

 know from the work of Spratt ( 1 948) that the embryo can grow and difTerentiate 

 in a medium containing only glucose as a carbon source. It is certain that Spratt's 

 explanted embryos undergo an extensive DNA synthesis : it is probable that they 

 are capable of a total DNA synthesis and that they are not dependent on a pre- 

 existing DNA reserve (Solomon, 1957). 



It should be added that recent work by Levenbook et al. (1955) has conclusively 

 shown that Drosophila eggs contain large amounts of free purines and pyrimidines, 

 ribosides, deoxyribosides (especially uracil deoxyriboside) which might be effec- 

 tive precursors of DNA (grasshopper eggs contain important amounts of DNA 

 [Durand, 1955]). Insect eggs might, however, prove to be a special case, because 

 of the absorption of nurse-cells, including their nuclei, by the growing oocyte. 



Much more experimental work is obviously needed before any conclusion can 

 be drawn : recent investigations suggest the presence, in unfertilized eggs, of rela- 

 tively large amounts of DNA precursors. They may range in complexity from 

 free purines, especially hypoxanthine, (Steinert, 1952, for frog eggs and Hultin, 

 1953, for sea urchin eggs) to large DNA-resembling molecules. It certainly will 

 be of considerable interest to know whether kinetin, the DNA derivative which 

 promotes cell division in plant tissues according to Miller et al. (1955), is also pres- 

 ent in unfertilized eggs. 



{b) RNA and morphogenesis 



This problem will just be outlined here, since the all-important question of 

 organizers will be treated by Dalcq (see Chapter 4) ; furthermore, the question of 

 the role of RNA in morphogenesis has been discussed at some length by Brachet 

 (1952) in a fairly recent monograph. 



Cytochemical observations on the distribution of RNA during development of 

 Amphibia show a distinct polarity gradient in unfertilized, fertilized and cleaving 

 eggs : the amount of RNA progressively decreases from the animal to the vegetal 

 pole. 



At gastrulation, a secondary RNA gradient, decreasing from dorsal to ventral 

 superimposes itself on the primary animal-vegetal gradient. The interaction of 

 the two, during gastrulation and neurulation, leads to the appearance of well- 

 defined cephalo- caudal (in the nervous system and the notochord) and dorso-ven- 

 tral (chordomesoblastic) RNA gradients. 



At still later stages, just before differentiation of the organ begins, a marked 

 increase in basophilia is conspicuous. 



The existence of the early vegetal-animal, dorso-ventral and cephalo-caudal 



