206 PRINCIPLES OF EMBRYOLOGY 



general, the most effective inducing region (the node) exhibits the highest 

 metabohc activity. But it is even more difficult for the chick than it is 

 for the amphibian to make any convincing suggestion as to which meta- 

 bohc peculiarities are mainly responsible for endowing the organiser 

 with its inducing capacity. 



5. What occurs during evocation? 



The study of the protein metabolism of the embryo is not only perhaps 

 relevant to the activation of the evocator at the blastopore hp, but is even 

 more likely to lead us into the centre of the problem of the nature of the 

 biochemical events concerned in evocation. The differentiation of tissues 

 certainly involves the appearance of tissue-specific proteins and it is 

 probable that an alteration in protein metabohsm is one of the most 

 important results of induction. It is, however, extremely difficult to detect 

 small differences in the protein constituents of cells and the analysis of the 

 protein metabolism connected with early differentiation and deter- 

 mination is as yet in its infancy. New techniques will probably have to be 

 elaborated before we can get very far. One of the methods now being 

 explored is paper chromatography. When the proteins of the embryo are 

 hydrolysed and the amino-acids assayed by this method no differences 

 were found either between different stages of development or between 

 different species (Holtfreter, Kozalka and Miller 1950; Eakin 1952). Using 

 fresh unhydrohsed tissues, Clayton (1954) has found considerable differ- 

 ences between tissues in fairly young mouse embryos but in these tissue 

 differentiation had already proceeded well beyond the point at which 

 evocator action occurs. Kutsky, Eakin, Berg and Kavanau (1953) have used 

 similar methods in whole amphibian embryos and found a definite 

 sequence of stages as development proceeds. It is probable that further 

 work will show that this method of analysis can give valuable results in 

 the analysis of different regions of the embryo. 



Another method which is beginning to be used depends on the incor- 

 poration into the proteins of amino-acids labelled with radioactive 

 isotopes. Eakin, Kutsky and Berg (195 1) have found that such amino- 

 acids are incorporated more rapidly into dorsal halves than in the ventral 

 halves of the gastrula ; and probably this means that proteins are being 

 synthesised more rapidly in the blastopore region. Sirlin and Wad- 

 dington (1954) have also found that the rate of incorporation varies from 

 tissue to tissue in the early neurula. These two authors, and also Ficq 

 (1954) discovered the interesting fact that the amino-acids are incorporated 

 more rapidly into nucleus than into the cytoplasm in these early stages of 

 development. This may indicate that protein synthesis is actually pro- 



