HEAD-FOLD TO TWELVE SOMITES 



109 



N.B 





op. Ves. 



■M.B. 

 FG. 



Hi. 







j 0;;-.o:-^-.'.-.Cr 



Ao. 







-3. IS. 



■■^ 



-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 liead-fold it con- 

 sists of two great sheets of cells between ectoderm and entoderm 



