I INTRODUCTORY 7 



coelomic epithelium, the development of hair follicles, the tropho- 

 blast in the Mammalian placenta (Fig-. 4). 





FIG. 2. Three stages of an imagination or evagination. 

 (After Korschelt and Heider.) 



FIG. 3. Displacement of a row of 

 cells in an epithelium. (After Kor- 

 schelt and Heider.) 



FIG. 4. Four stages in the formation 

 of an epithelial thickening of many 

 layers. (After Korschelt and Heider.) 



b. Decrease : thinnings : e. g. in the roof of the thalam- 

 encephalon and medulla, the outer layer of the lens, the tropho- 

 blast of the Mammalian blastocyst. 



