INTRODUCTORY 



9 



iv. Concrescence of layers. 



a. By their margins : e. g. the edges o the ectoderm over 

 the medullary plate, the edges of the embryonic ectoderm inside 



A 







FIG. 7. Fusion of two cell plates by their margins. 

 (After Korschelt and Heider.) 



the serosa of Sipunculus, the embryonic plate with the tropho- 

 blast in some Mammals (Figs. 7, 9). 



b. By their surfaces (Figs. 8, 9, 10) : e. g. when the stomo- 

 daeum or proctodaeum open into the gut, when the medullary 



A 



FIG. 8. Fusion of two cell plates by their surfaces. 

 (After Korschelt and Heider.) 



folds meet, when the edges of the peritoneal groove close to form 

 the canal of the oviduct in Amphibia and Amniota. 



This concrescence is commonly followed by a communication 

 of the cavities on opposite sides of the adherent layers, as when 

 the stomodaeum opens into the gut, or the amnion-folds unite ; 

 but not necessarily, as when the somatopleure fuses with the 

 trophoblast, or the allantois with the somatopleure in Mammalia. 



v. Splitting of a layer into two : e. g. the inner wall of the 

 pineal vesicle in Lacertilia (Fig. 11). 



