HEAD-FOLD TO TWELVE SOMITES 



109 



Ji.B. 



Vo.m. 



^.2. 



3.JZ. 



■pr str. 



Fig. 63. — Embryo of 12 s, from above, drawn 

 as a transparent object with transmitted 

 light. X 30. Abbreviations as before. 



IV. The Mesoblast 



The changes in the mesoblast during this period are of great 

 importance. At the time of appearance of the head-fold it con- 

 sists of two great sheets of cells between ectoderm and entoderm 



