DIFFERENTIATION OF NERVOUS STRUCTURE 109 



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FIGS. 13-15. Axial gradients in development of the central nervous 

 system: Fig. 13, embryo of Plagiostomum girardi, a rhabdocoel turbel- 

 larian; longitudinal section near median plane with anterior end at 

 left; the mass of cells anterior to the pharynx represents the cephalic 

 ganglia and one eye. These organs are advanced in development, but 

 postcephalic regions have scarcely begun to develop (from Bresslau, 

 1904) ; Figs. 14, 15, surface views of two stages in development of chick, 

 showing progress of development in posterior direction. In Fig. 14 the 

 central nervous system is marked off as a distinct region, the medullary 

 plate, only in the anterior head region. In Fig. 15 the neural tube has 

 developed by closure of the medullary folds to a level posterior to the 

 somites, 5. Still farther posterior is the secondary region of growth 

 (from F. R. Lillie, 1919). 



