50 EARLY AMPHIBIAN DEVELOPMENT 



differentiation which they would not normally have carried out, this 

 plasticity does not mean that the regions are entirely indifferent. 

 On the contrary, experiments have shown that even at the start of 

 gastrulation in the newt, there is a feeble determination of the 

 presumptive neural tube region, in virtue of which it tends to 

 differentiate along the lines of its prospective fate,^ and the same 

 is true of other regions, as will be seen later (pp. 136, 203, figs. 18, 

 62,63, 64)- 



§6 



There is, however, one region of the amphibian embryo which 

 makes a very important exception to the statement that the tissues 

 at early stages are plastic. This is the region of the dorsal lip of the 

 blastopore, which has arisen from the grey crescent and is destined 

 to form the notochord and mesoderm (chorda-mesoderm). This is 

 determined from very early stages (possibly even in the fertilised 

 egg before cleavage has begun). When grafted into other embryos 

 h will differentiate in no direction other than that of its normal 

 presumptive fate.^ 



This presumptive notochord, gut-roof, and mesoderm region is 

 predetermined to invaginate beneath the surface. It has other 

 properties which are as remarkable as they are important. If a portion 

 of this region be grafted into another embryo in the blastula or early 

 gastrula stage and in any position, it will there pass below the sur- 

 face and proceed to induce the neighbouring host-tissues to under- 

 go differentiation into the main organs of an embryo, often including 

 neural tube and brain, eyes and ears, spinal cord, mesodermal 

 somites and pronephric tubules, quite regardless of what the pre- 

 sumptive fates of these host-tissues may have been. In other words, 

 the dorsal lip of the amphibian blastopore has the property of being 

 able to force other tissues (during their state of plasticity) to under- 

 go the organised differentiations and developments which lead to 

 the production of an embryo. For this reason, the dorsal lip of the 

 blastopore has received the name of organiser, as the German term 

 Organisator coined by Spemann may be translated^ (^gs- 1? i9> 65). 



^ Goerttler, 1926; Holtfreter, 193 1 a. 



" In certain conditions, as when cultivated in vitro, etc., it may give rise to 

 other organs, such as nervous system and gut (Holtfreter, 193 1 a). 

 ^ Spemann and Mangold,. H., 1924. 



