CHAPTER XIV. 



THE ORGANS OF THE INNER GERM-LAYER. 

 THE ALIMENTABY TUBE WITH ITS APPENDED OBGANS. 



AFTER completion of the formation of the germ-layers and the first 

 processes of differentiation described in the tenth chapter, the body 

 of the vertebrated animal consists of two simple tubes, one within 

 the other (Plate I., figs. 7 and 10), the inner, smaller alimentary 

 tube, and the body-tube separated from the former by the body- 

 cavity (lh'}, each of which is composed of more than one of the 

 primitive cell-layers of the germ. 



The alimentary tube, the further development of which will first 

 engage our attention, is composed of two epithelial layers, the 

 entoderm and the visceral portion of the middle layer, which fur- 

 nishes the epithelial lining of the body -cavity, separated from each 

 other by the intermediate layer, which is at this time little developed. 

 Of the three layers the entoderm is unquestionably the most im- 

 portant, since the further processes of differentiation primarily 

 proceed from it, and since the physiological capabilities of the 

 alimentary canal are determined by the activity of its cells. 



The changes which occur in the further course of development are 

 best divided into three groups. First, the alimentary tube comes 

 into communication with the surface of the body by means of a large 

 number of openings, the visceral clefts, the mouth, and the anus. 

 Secondly, it grows enormously in length, and is at the same time 

 differentiated into oesophagus, stomach, small intestine and large 

 intestine, with their peculiarly modified mesenteries and omenta. 

 Thirdly, numerous organs, which are for the most part concerned 

 in the duties of digestion, take their origin from the walls of the 

 alimentary tube. 



