THE FOUMATION OF THE LAYERS IN AMFHIOXUS. 345 
cavity has acquired an opening to the exterior, slightly older than that 
represented in fig. 16. In fig. 17 « the left head-cavity is seen opening 
to the exterior. In fig. 17 b the collar-cavities are seen extending do^wn at 
the sides of the pharynx, and the external opening of the clnh-shaped 
gland is seen at club in front of the mouth. In fig. 17 c the mouth is seen, 
and on the j)osterior extension of the right collar-cavity (r.coll.) an 
ectodermal thickening (at.), the rudiment of the atriiil ridge. Figs. 
1 7 d, e, /, fj, and h are five consecutive sections through the hinder end 
of the pharynx hehind the gill-slit, which is grazed in fig. 17 d (br.end.) 
to show how the extensions of the collar-cavities thin out and disappear, 
whilst right and left .splanchnoceles meet in the mid-ventral line. 
Magnification 1000 diameters. 
