HEAD-FOLD TO TWELVE SOMITES 



109 



Vo.m. 



3. te. 



- -pr. st-f. 



FIG. 63. Embryo of 12 s, from above, drawn 

 as a transparent object with transmitted *\t 

 light, x 30. Abbreviations as before. 7) 



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 



