86 



EMBRYOLOGY 



Since many compounds stimulate the ectoderm of the early gastrula to 

 form a neural tube, we may well assume a release mechanism for the explana- 

 tion of this induction. In seeking for a theory of fertilization we assumed that 

 the necessary chemical substances for development were present in the egg 

 but were masked or blocked in some way and were released by sperm or by 

 chemical agents. In the present ca§e, too, we think that the ectoderm con- 

 tains a substance which, when released by the chemical stimuli, will bring 

 about reactions leading to the formation of the primitive nerve cells and 

 finally the neural tube. We may picture an ectoderm cell in the early gastrula 

 as containing compounds which in some way are masked or blocked from 

 reacting (Fig. 40). Then either an artificial chemical stimulus or the natural 

 living organizer from the roof of the archenteron diffuses into this cell, 

 liberates the previously inactive compound, and thus starts a chain of reactions 

 which convert the cell into the typical elongated primitive nerve cell. 



Once this liberation or activation takes place the ectoderm cell becomes 



BLOCKING 



LAYER 



\ 



ACTIVE— ^)0 00 

 SUBSTANCE 



TT 



ECTODERM 

 CELL 



OF 



EARLY 



GASTRULA 



LIVING ORGANIZER 

 OR CHEMICAL COMPOUNDS 



ECTODERM 



CELL AT 



END OF 



GASTRULATION 



ACTIVE SUBSTANCE LIBERATED 



Fig. 40. Above: In the undifferentiated ecto- 

 derm cell in the early gastrula, the active substance 

 necessary for the differentiation of a nerve cell is as- 

 sumed to be covered by a blocking layer. When the 

 blocking layer is removed, the active substance is 

 liberated. Right: The cell becomes a primitive 

 nerve cell. When this cell divides, the two daugh- 

 ter cells must also contain the active substance. 



NEURAL PLATE 



CELL 



NEURULA 



ACTIVE 



SUBSTANCE 

 REPRODUCING 



/ \ 



• • 



CELL 

 DIVISION 



OF 



NEURAL 



CELL 



IN 



NEURULA 



