I. NEURULATION 107 



cells of the animal field were grafted into the dorsal marginal 

 zone. In this material, too, glycogen decomposition took place 

 when it was rolled in over the lip of the blastopore, even though 

 in normal development it would have stayed at the surface, 

 and retained its glycogen. Thirdly, the same material did not 

 lose its glycogen if brought directly into the interior of the 

 germ through a slit cut in the roof of the blastula (Raven, 

 1933-35). Finally, marginal zone material does not lose its 

 glycogen when cultured in isolation from the rest of the embryo 

 (Jaeger, 1945). Hence, the disappearance of the glycogen pre- 

 sumably is a direct consequence of the rolling in of the cells 

 during gastrulation. 



In this context mention must also be made of observations 

 on the distribution of ribonucleic acid and sulphydryl com- 

 pounds (Brachet, 1940-42), and of alkaline phosphatase (Kruge- 

 lis, 1947) in amphibian embryos. In each case, a more homo- 

 geneous distribution, obtaining in the younger stages, was seen 

 to be replaced during gastrulation by a distribution in which 

 there were distinct differences in concentration among the 

 various parts of the germ. 



However, such observations on changes in the physico- 

 chemical constitution of the cells are rare. In most cases, the 

 occurrence of chemodifferentiation cannot be demonstrated 

 directly; it appears only indirectly from changes in the 

 potencies of the cells during gastrulation. We possess more 

 accurate knowledge on this point, particularly in amphibians. 



The histogenetic potencies, or potentiality for differentiation, 

 of the cells can be studied by removing a small group of cells 

 from the germ, and rearing it in isolation in an appropriate salt 

 solution. This method is called explantation. TroMsplantation, 

 the grafting of the removed cell group into another embryo, 

 is another useful method. In the case of explantation, we may 

 generally assume that the cells are not exposed to any specific 

 modifying influences on the differentiation processes. Such 

 experiments inform us what the cells concerned can do "on 

 their own", and therefore give us an insight into their auto- 

 nomous powers of differentiation. Cells transplanted into an- 

 other embryo will there be subjected to the action of the 



