8 



INTRODUCTORY 



iii. Interruptions of continuity. 



a. By the atrophy of part of a layer : e. g. when the floor 

 of the archenteron together with the underlying paraderm dis- 

 appears in Amniota (Fig. 5). 



A 



e 



FIG. 5. Three stages in the development of an interruption of con- 

 tinuity perpendicular to the surface of an epithelium. (Perforation.) 

 (After Korschelt and Heider.) 



b. By the detachment of a part: e.g. of the medullary 

 plate from the ectoderm in AmpMoxus (Fig. 6), of the notochord 

 from the roof of the archenteron in Urodela and Pelromyzon. 



A 





FIG. 6. Scheme of the formation of the medullary canal in 

 Ainphioxus. (After Korschelt and Heider.) 



