HEAD-FOLD TO TWELVE SOMITES 109 
Embryo of 12 s, from above, drawn 
Fia. 63. 
as a transparent object with transmitted 
light. x 30. Abbreviations as before. 
IV. Tue 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 
