90 



EMBRYOLOGY 



ORGANIZER = * N INDUCTORS 



ORGANIZER ■ GRADED CONCENTRATION OF 



Fig. 42. Schematic representations of two theories regarding the nature of 

 the organizer. Left: A number of different chemical substances are assumed 

 to be responsible for the induction of different parts of the brain and spinal 

 cord. l FB , a hypothetical substance, acts on the ectoderm and converts it into 

 forebrain ; I HB converts it into hindbrain ; I sc converts it into spinal cord. The 

 organizer would then be the summation (2N) of all the individual inductors 

 (/ substances). 



Right: One substance or factor, O, is assumed to be responsible for the 

 induction of the nervous system. The various parts of the nervous system form 

 in response to various concentrations of this substance or factor. C t is a concen- 

 tration of O which induces forebrain; C % induces hindbrain; C 3 induces spinal 

 cord. 



more posterior. The neural plate is induced above regions 1 through 5. The 

 head organizer corresponds to about region 2, and the trunk organizer to 

 region 4. Thus in normal development it would be logical to conclude that 

 there is a difference between region 2, which induces ectoderm to develop 

 into head structures, and region 4, which brings about the formation of 

 trunk structures. 



Theories of organization 



The origin and nature of these differences between the parts of the organ- 

 izer are very difficult to seize upon. The same two kinds of explanation out- 

 lined earlier for the organization of the sea urchin egg may be applied to the 

 nature of the organizer. There may be qualitative differences within the 

 roof of the archenteron, or there may be quantitative differences between 

 the head and trunk regions of the organizer. 



