108 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



: i- s.-ctimis 

 tin- ilcvi-lnpiii.-nt iif ill,- spinal in-i\, \mlial ..... 



I' ni\..ti.iin- ; -.. ii. ! M-II unk ; >... ^pmal -nnl ; sh, 



to illu-t rate 



