66 GROWTH AND MORPHOGENESIS 



with the microsomes, along gradients which superimpose them- 

 selves on the morphogenetic gradients. One can, therefore, hardly 

 avoid the conclusion that they represent regions where the yolk 

 reserves are transformed into *'true" cytoplasm (ergastoplasm and 

 mitochondria) at a faster rate than elsewhere. From another angle, 

 these gradients should be considered as gradients in the 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), which clearly show that after gastrula- 

 tion the structure of the mitochondria and the ergastoplasm be- 

 comes more and more complicated as differentiation 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 neighbouring cells. 

 These results have been confirmed by Spratt (1952), who used to- 

 luidine blue as a stain for RNA detection. We have already seen 

 that in the chick as well as in the amphibian egg, increased baso- 

 philia is found at the interface between neuroblast and chordomeso- 

 blast, and thus at the very site of induction (Lavarack, 1957). 



Gradients in RNA distribution, which are essentially similar to 

 those described for the amphibians, have also been observed in 

 embryos of the fishes (Brachet, 1940, 1942) and the reptiles. 



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

 which have been obtained with mammalian eggs, because the 

 latter are too different from those of the other vertebrates. A few 

 words should, however, be said about the very interesting results 

 obtained by Dalcq and his co-workers on mammalian material. 

 They have been very adequately summarized by Dalcq himself in a 

 recent book (1957). 



The cytochemical studies of Dalcq and his school have clearly 

 shown that definite patterns in RNA distribution and synthesis 



