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 navel 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 wall, 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 whole, arises, therefore, from the entoderm, and 

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

 the exoderm. 95 



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, 



