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THE ORGANIZATION CENTRE 7 1 



blastopore." Instead, young embryos were taken at a 

 stage when the primitive streak or thickening had 

 just appeared ; the endoderm was removed, and 

 then replaced after being turned round through 

 1 80 degrees, so that the part which originally lay 

 under the primitive streak in the posterior region 

 near the endoderm-blastopore now lay under the 

 other, anterior end. In some of these operated 

 embryos an extra primitive streak appeared in the 

 anterior end, where it had been induced by the 

 endoderm coming from the endoderm-blastopore 

 region, which is therefore an organization centre. 

 Meanwhile the original primitive streak, which had 

 been induced by the endoderm before the operation 

 was made, continued to develop, so that finally two 

 embryos appeared on the one blastoderm. 



The next experiment dealt with the formation of 

 the mesoderm from the primitive streak at a rather 

 later stage. It is easy to take a piece of primitive 

 streak from one embryo and place it between the 

 ectoderm and endoderm of another; it develops 

 by self-differentiation in its new situation into its 

 presumptive fate, which is mesoderm and a little 

 neural plate tissue. But the important thing is that 

 the host ectoderm lying above it, which ought to turn 

 into skin or part of a yolk-absorbing structure, now is 

 induced to form a secondary embryo with a neural 

 plate and other organs. This proves that at least some 

 of the presumptive mesoderm, that part which lies at 

 the front end of the primitive streak, is also an organ- 

 ization centre (Fig. i8), as it is in the newt embryo. 



