204 



PRINCIPLES OF EMBRYOLOGY 



embryos treated with radioactive amino-acids, that these are incor- 

 porated'more rapidly into the nuclei than into the cytoplasm at the early 

 gastrula stage, and that this incorporation, which is probably a sign of 

 synthetic processes, begins first in the dorsal lip region (Fig. 10.15). 



A little later, when gastrulation is^imder way, there seems to be an 

 increase in the cytoplasmic RNA. This can be demonstrated by measuring 



Figure 10.15 



On" the left, three diagrammatic sections through the head process (above) 

 the node, and the streak (below) in a chick embryo treated with radioactive 

 methionine. The shading indicates the relative concentration of the tracer, 

 as judged from autoradiographs. (After Feldman and Waddington 1955.) 

 On the right, an autoradiograph of a section through a newt gastrula culti- 

 vatedin a solution containing radioactive methionine. The amino-acidhas been 

 incorporated into the proteins, and probably the nucleic acids, particularly 

 in the nuclei of the blastopore region. (After Sirlin, I955-) 



the passage of radioactive phosphorus P^^ into the RNA fraction of the 

 egg (Kutsky 1950) as well as by histochemical methods. The latter show 

 that the process is not uniform throughout the gastrula, but that the RNA 

 is at first concentrated in the blastopore region. Histochemical methods 

 which reveal protein containing -SH groups give an almost identical 

 picture (Brachet 1944). 



Less is known about the metabolism of different regions of the chick 

 blastoderm at the time when the primitive streak is active as an organiser 

 (Reviews: Waddington 1952, Spratt 1952^). Direct measurements of 



