THE "organization GENTRE" 69 



weaker further away from it. Spemann gave the 

 name of organizer to pieces of tissue which can 

 determine in this way, and of organization centre to 

 the part of the embryo where they occur, but these 

 two names are often used more or less interchange- 

 ably in a rather loose way. The organization centre 

 is the whole presumptive mesoderm, which, as we 

 know, sinks in through the blastopore and grows 

 forward along the inside. Finally the mesoderm 

 spreads over the entire embryo between the ectoderm 

 and the endoderm, but at first it forms a narrow 

 tongue which is the roof of the primitive gut, and it 

 is in this stage that it determines the ectoderm lying 

 immediately above it to become neural plate, while 

 all the rest becomes skin. 



The importance of the organization centre can 

 be exhibited in two ways. One way is to point out 

 that it causes part of the gastrula-ectoderm to 

 develop into neural plate. This property is important 

 enough, but other similar development-provoking 

 agents, or determiners, had previously been dis- 

 covered. The special importance of the organization 

 centre is better conveyed by the name Spemann 

 actually chose ; it is that part of the embryo with 

 respect to which all the rest is organized. In order 

 to describe the behaviour of any part of a newt 

 gastrula, it is necessary and sufficient to specify 

 its relation to the organization centre. Spemann' s 

 name for his discovery may at first sight seem 

 rather grandiloquent, but is really quite reasonable 

 and accurate. 



