158 DEVELOPMENT OF AMPHIOXUS. 



Thus the actual topographical conditions in the larva do 

 not by any means coincide with the morphological rela- 

 tions of parts, since the morphological mid-ventral line of 

 the pharynx lies high up on the right side of the body. It 

 should be carefully noted that the form of asymmetry 

 which we are now considering only affects the anterior 

 portion of the larval body. 



The same semi-rotation of the pharyngeal region which 

 converted the primitive left side of the larva into the 

 actual right side caused the primitively median mouth to 

 take up its position on the actual left side. But since, as 

 we have noted, the rotation occurred from left to right, 

 the mouth must have been originally situated in the 

 median dorsal line. 



In postulating a virtual semi-rotation of the ancestral 

 pharynx, we do not, of course, mean to suggest the prob- 

 ability of an actual movement in bulk about the longi- 

 tudinal axis, but merely that the formative centres of the 

 various structures belonging to this region of the body 

 (gill-slits, mouth, endostyle, etc.) have, by the correlated 

 interaction of their component cell-groups, been diverted 

 from their ancestral relations through the intercalation, in 

 the course of the progressive evolution of the organism, of 

 a new and disturbing element. 



We are now in a position to say what this disturbing 

 element is. It is the secondary forward extension of the 

 notochord beyond the limits of the dorsal nerve-tube to 

 the tip of the snout. As already stated, there is direct 

 evidence to show that this is a secondary and not an an- 

 cestral feature, inasmuch as in the young embryo (Fig. 

 68 bis} the notochord is removed from the anterior extrem- 

 ity of the body by a very appreciable interval, which is oc- 

 cupied by that portion of the archenteron which gives rise 



