248 CELLS, TISSUES, AND ORGANISMS 



distribution of such cytoplasmic fractions as RNA-rich ergastoplasmic 

 granules or vesicles, and mitochondria. Such a conclusion is reinforced 

 by the recent electron-microscope studies of Karasaki ( 1959 ) ; they 

 clearly show that, after gastrulation, the structure of the mitochondria 

 and the ergastoplasm becomes more and more complicated as differ- 

 entiation progresses. 



Before we can consider these gradients as important factors in 

 morphogenesis, one important question should be answered: Are there 

 similar gradients in vertebrate eggs other than those of the amphibians? 



The most complete study of RNA distribution in chick embryos is 

 that of Gallera and Oprecht ( 1948 ) , who showed that node center cells 

 exhibit greater cytoplasmic basophilia than neighboring cells; these re- 

 sults have been confirmed by Spratt ( 1952 ) , who used toluidine blue 

 as a stain for RNA detection. 



Gradients in RNA distribution essentially similar to those de- 

 scribed for the amphibians have also been observed in embryos of the 

 fishes (Brachet, 1940) and the reptiles (Pasteels, 1953). 



It is beyond the scope of the present review to present the results 

 obtained with mammalian eggs, because they are too different from 

 those of the other vertebrates. It can be said, however, that the cyto- 

 chemical studies of Dalcq and his school ( 1957 ) have clearly shown 

 that definite patterns in RNA distribution and synthesis occur during 

 early development of mammalian eggs, and that RNA synthesis in 

 them, as in other vertebrates, is especially marked in the mesoblast and 

 the induced parts of the ectoblast. 



In short, the cytochemical data obtained in the cases of fishes, rep- 

 tiles, birds, and mammals agree very well with the general conclusions 

 we have drawn from the study of amphibian eggs: that RNA is ac- 

 cumulated and is most actively synthesized in the regions of the embryo 

 which have the greatest importance for morphogenetic processes. 



We shall now try to answer another important question: What 

 happens to the ribonucleoprotein gradients when morphogenesis or 

 RNA synthesis is experimentally modified? 



Experimental effects on RNA gradients in amphibian eggs 



If synthesis of RNA along animal and vegetal gradients is really an 

 essential factor in morphogenesis, inhibition of RNA synthesis by treat- 

 ment with chemical analogues of purines and pyrimidines should lead 

 to the cessation of development or to abnormal development. 



This expectation has been fulfilled, as was first shown by the 

 author (1944) in the cases of barbituric acid, benzimidazole, and acri- 

 flavine. These early studies have been considerably extended by Bieber 

 (1954), Bieber and Hitchings (1955), and Liedke et al. (1954, 1957 



