480 J. BRACKET 



Recent work with P^^ by Kiitsky^" also shows that phosphate metaboHsm 

 is very sluggish during cleavage; but, at gastrulation, there is a transfer of 

 the labile P of ATP to PNA, the specific activity of which markedly in- 

 creases. This faster uptake of the PNA-phosphorus might well be linked 

 together with inductive processes or increased protein synthesis at that 

 stage. 



It should be added, however, that gradients similar to those which have 

 just been described for PNA also occur for other substances: — SH groups 

 linked to the proteins (Brachet^'), reducing power (Piepho,- Fischer and 

 Hartwig,^^ Child,^'') and alkaline phosphatase (Krugelis-^). These findings 

 suggest that the morphogenetic gradients are, in fact, gradients in the 

 distribution of cytoplasmic particles (microsomes and mitochondria). 



It should be pointed out, finally, that PNA gradients parallel to morpho- 

 genetic gradients have also been found in fishes and birds (Brachet,-^ Pas- 

 teels," Gallera and Oprecht,^'' etc.), as well as in mammals (Dalcq and 

 Seaton-Jones^'). 



There is thus little doubt that distribution and local synthesis of PNA 

 are important in normal embryonic development: In what way can experi- 

 ments in which either PNA metabolism or morphogenesis have been arti- 

 ficially altered provide evidence in support of this view? 



2. Experimental Modifications of PNA Synthesis or Distribution: 

 Effects on Morphogenesis 



It is convenient to deal successively with the effects on PNA synthesis 

 and morphogenesis produced by chemicals (some of which are thought to 

 act specifically on nucleic acids metabolism), by physical agents (heat, 

 centrifugation), and by nuclear abnormalities. 



a. Chemical Analogues of Purines and Pyrimidines 



It has been shown that chemical analogues of purines and pyrimidines, 

 barbituric acid and benzimidazole, for instance, slow down considerably or 

 even stop completely both morphogenesis and PNA synthesis (Brachet'^); 

 the effect is perfectly reversible and further development is normal. Similar 



20 P. B. Kutsky, /. Expll. Zool. 115, 429 (1950). 



21 J. Brachet, Arch. hiol. {Liege) 51, 167 (1940). 

 " H. Piepho, Biol. Zentr. 58, 90 (1938). 



" F. G. Fischer and H. Hartwig, Z. vergleich. Physiol. 24, 1 (1936). 

 " C. M. Child, J. Expil. Zool. 109, 79 (1948). 



26 E. Krugelis, Biol. Bull. 93, 215 (1947). 



2« J. Brachet, Arch. hiol. {Liege) 53, 207 (1942). 



27 J. Pasteels, Arch. biol. {Liege) 60, 235 (1949). 



28 J. Gallera and O. Oprecht, Rev. Suisse zool. 55, 243 (1948). 



2' A. Dalcq and A. Seaton-Jones, Bull. acad. roy. Belg. 35, 500 (1949). 



I 



