SEGMENTATION 



139 



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 42 **!&& 



FIGS. 40-44. Embryonic segmentation of the central nervous sys- 

 tem of vertebrates: Figs. 40, 41, Squalus acanthias, dogfish, surface 

 view of young embryo and side view of nervous system of later stage 

 (from Locy, 1895); Figs. 42-44, chick; Fig. 42, dorsal view of embryo 

 with one somite, twenty-one hours old, neural segments traceable, not 

 only across wide neural groove but some distance laterally into adjacent 

 tissue; Fig. 43, dorsal view of anterior portion of neural tube of embryo 

 twenty-two hours old; Fig. 44, right surface view of same neural tube 

 as Fig. 43, showing that segments encircle neural axis (from Hill, 1900). 



