10 



INTRODUCTORY 



FIG. 9. Diagram to illustrate 

 the formation of the fore-gut (stoin- 

 odaeuin) and its opening into the 

 mid-gut : ec, ectoderm ; md, mid- 

 gut ; vd, stomodaeum. (After Kor- 

 schelt and Heider.) 



FIG. 10. Scheme of the formation of 

 the medullary canal in a Vertebrate. 

 (After Korschelt and Heider.) 



FIG 11. Three stages in the 

 development of an interruption 

 of continuity parallel to the sur- 

 face of an epithelium. (Dela- 

 mination.) (After Korschelt 

 and Heider.) 





