HEAD-FOLD TO TWELVE SOIMITES 113 



but this arises normally at the stage of 12-14 somites, a very 

 short distance in front of the first somite of the series, which is 

 thus shown to have the same position as the first somite formed. 

 On the other hand, if one assumed that the first somite formed 



/y.F 





-r 



Ht. 



'A.C.V. 



^'cA ^. 



yo.m. 



^.i.p. 



'•'->>^-<^>>"Ci" 



tfV*^'- -rf.-'-i 



^"^^'TT, V '','' '^"'' '^"''"■y" '■'■°'" '^<=°<'^t"'' drawn in alcohol with 

 reflected light. Abbreviations as before. 



became the third or fourth of the series, it is clear that one would 

 Zll^yT^-^ mark some distance in front of the first somite 

 formed, to strike the place of origin of the otocyst. Marks made 



frlr f ., T '''"■'' '''"'^^' ^"""^ "- ''""^Werable distance in 

 tront of the otocyst. Altogether a large number of experiments 



