SELF-DIFFERENTIATION OF THE PARTS OF THE NEURULA 91 



Some embryologists suggest that there are special substances, inductors, 

 which may be called / substances; these are believed to be present in the 

 roof of the archenteron and they are thought to be responsible for the stimu- 

 lation of each part of the nervous system. There would thus be a compound 

 1 PB for the stimulation of the forebrain; 1 HB for the hindbrain; and I ac for 

 the spinal cord (Fig. 42). The theory would demand a host of these com- 

 pounds, each inducing a particular region of the nervous system. 



Other embryologists believe that there is no justification for assuming 

 this complex system of inductors, that the same effect could be produced if 

 only one compound, O, is assumed. This compound may be present in different 

 concentrations, such that in the head region the concentration would be G; 

 a little farther back, Ci\ and so on through a wide range of different concen- 

 trations of a single substance (Fig. 42). Adherents of this theory point out 

 that in the early stages of differentiation of the nervous system there are no 

 visible differences in the primitive neural tube, so that the striking differences 

 which appear later in development arise from a neural tube which shows 

 only quantitative differences. They say that these quantitative differences in 

 the early nervous system could be brought about by quantitative differences in 

 the inductor in the roof of the archenteron. 



There is no way at present of deciding which of these two theories is 

 more nearly correct, for we do not have the necessary facts. We do not know 

 the chemical nature of the substance in the organizer and until we do we 

 cannot test the effect of graded concentrations of it in order to determine 

 whether different concentrations produce different effects. 



Self-differentiation of the parts of 

 the neurula 



Let us continue with our analysis of development. After the organizer 

 has induced the neural plate, how do the different structures of the body 

 arise? In general, we have already had indications that individual structures 

 are not determined in the early gastrula stage. Eye, ear, and nasal placode 

 will not differentiate if taken from the early gastrula and transplanted to 

 some other region. However, at the stage when the neural tube has formed, 

 the neurula stage (Fig. 43), almost every major structure is determined and 

 localized — even though there are no visible signs of the structure. This deter- 



