DEVELOPMENT OF THE DORSAL AND VENTRAL WALLS. 317 



these grow toward each other from all sides, and at last 

 unite in the navel at the centre. We must, therefore, dis- 

 tinguish between two navels, an inner and an outer. The 

 inner or intestinal nav^el is the point at which the in- 

 testinal wall finally closes, at which the communication 

 between the intestinal cavity and the cavity of the yelk- 

 sac was cut off (Fig. 70). The outer or skin-navel is the 

 point at which the ventral wall finally closes, and which 

 even in adults is visible as a depression. In each concrescence 

 two secondary germ-layers are concerned; at that of the 

 intestinal v^all, the intestinal-glandular layer and the in- 

 testinal-fibrous layer ; at that of the ventral wall, the skin- 

 fibrous layer and the skin-sensory layer. The intestinal 

 wall, as a wdaole, arises, therefore, from the entoderm, and 

 the ventral wall (and, indeed, the entire body-wall) from 

 the exoderm.^^ 



The processes by which the double tubular rudiment of 

 the body originates from the four-layered germ-disc are, 

 therefore, really very simple. They are not, however, at 

 once easily understood, nor is it easy to describe them. 

 Very much, doubtless, yet remains obscure to the reader, 



m. r- 



Fig. 95. 



