46 EARLY AMPHIBIAN DEVELOPMENT 



therefore be looked for in some factor which affects the various 

 regions of the cytoplasm in general, and not of the nucleus alone. 



By the method of transplantation it can be shown that, up to a 

 certain stage in the gastrulation of the newt, the fates of most of the 

 regions of the embryo are not irrevocably determined. A piece of 

 presumptive neural tube material removed from its embryo and 

 grafted into the side of another, may differentiate into the external 

 gills of its new host if it happened to be grafted into the presumptive 

 gill region of the latter. Conversely, a piece of presumptive epi- 

 dermis grafted into the appropriate region of the presumptive 

 neural tube of another embryo, will undergo differentiation into 

 part of the brain and the eye. Up to this stage of gastrulation, there- 

 fore, the regions develop according to their actual surroundings, 

 and regardless of their origin and former surroundings:^ they are 

 in fact still plastic as regards their final fate. Even the future germ- 

 layers are plastic up to this stage, for presumptive epidermis can 

 be made to differentiate into mesodermal structures such as muscle 

 fibres, and vice versa (figs. 14, 15 and 16).^ 



There comes a critical time, however, during the process of 

 gastrulation, after which the various presumptive regions are no 

 longer plastic. Their fates are then irrevocably determined, and, 

 whatever the position into which they may be grafted, pieces of 

 any given presumptive region will then undergo the differentiation 

 which is typical of that particular region in normal development. 

 Pari passu with the determination to differentiate in any given 

 direction goes the loss of power to differentiate in other directions. 

 In other words, the regions can then only develop towards their 

 presumptive fates. One can then, for instance, graft the presump- 

 tive eye region from one late gastrula into another, and obtain the 

 differentiation of a typical eye, facing into the body cavity (fig. 17)^. 

 Something invisible has happened to fix the prospective ^ates on 

 the various presumptive regions, and since this something must be 

 due, presumably, to chemical changes in the various regions, this 

 phase of development may be referred to as chemo-dijferentiation^ 

 Through this process the organism has become a patchwork or 

 mosaic of separately determined regions. It is of some interest to 



1 Spemann, 1918. - Mangold, 1924. ^ Spemann, 1919. 



* Huxley, 1924; Goldschmidt, 1927; Bertalanffy, 1928. 



