THE ORGANS OF THE INNER GERM-LAYER. 



291 



and when these are closed, it no longer leads directly to the outside, but 

 into the posterior end of the neural tube. It has thereby become 

 the familiar canalis neurentericus (fig. 159 ne). Neural tube and 

 intestinal canal together form a U-shaped tube, at the bend of 

 which the rudiment of the primitive mouth, or primitive groove, is 

 to be sought. 



The anus is a new structure. It arises on the ventral side of the 



Fig. 158.- Human embryo of the middle of the fifth week 9 mm. long, neck-rump measurement, 



after RABL. 

 , Mid-brain [parietal] elevation ; au, eye ; ok, upper jaw ; uk, lower jaw ; zb, hyoidarch ; hb, sinus 



cervicalis ; ng, nasal pit ; oe, anterior, ue, posterior limb ; mp, muscle plates (trunk-segments). 



body (fig. 159 an) at some distance in front of the place where the 

 neural tube bends around into the intestine. Over a small area 

 the entoderm and the epidermis here grow toward each other, 

 and, by crowding aside the middle germ-layer, come into contact and 

 form a thin septum, the anal membrane. Externally this place is 

 characterised in many animals by a depression of the epidermis, the 

 anal pit (fig. 159 an). The opening of the intestine to the outside 

 takes place in most cases at a rather advanced stage of development 

 by the rupture of the thin anal membrane, which consists of only 

 two epithelial layers. The process is therefore similar to that by 

 which the mouth is formed. In one important point, however, there 

 exists a difference between the opening at the anterior and that at 



