Ill 



THE ALIMENTARY CANAL 



185 



place in development causing 

 the lung rudiment to shift 

 dorsally round the right side of 

 the alimentary canal. 



In the more richly yolked 

 Vertebrates the ventral portions 

 of the gut-wall are more and 

 more clogged up with yolk and 

 this results in a greater and 

 greater concentration of de- 

 velopmental activity in the 

 dorsal wall. This is clearly in- 

 dicated by transverse sections 

 through the developing gut of 

 Vertebrates which though rich 

 in yolk are still holoblastic. 

 Such sections (Fig. 107) show 

 the dorsal wall of the gut to 

 consist of small active cells ar- 

 ranged as a columnar epithelium, 

 while the side walls and floor 

 consist of large comparatively 

 inert yolk -laden elements. It 

 is only as development goes on, 

 and as the yolk is consumed, 

 that the epithelial small-celled 

 character gradually spreads 

 ventrally. 



In the actually meroblastic 

 Vertebrates, the heavily yolked 

 portions of the primitive gut- 

 wall never undergo segmenta- 

 tion at all, unless possibly as 

 regards a thin superficial layer. 

 They remain as a continuous 

 mass of yolk, round which the 

 epithelium gradually spreads. 

 In this case the formation of all 

 the important organs of the ali- 

 mentary canal is concentrated 

 in the dorsal portion which be- 

 comes gradually folded off from 

 the main mass of the yolk. This 

 folding -off process takes place 

 most actively in the anterior 

 region, so as to form the tubular 

 fore -gut, and also posteriorly, 

 the intermediate portion re- 



