50 OUTLINES OF CHORDATE DEVELOPMENT 



(in these cases the median fins), is not unusual or of particular 

 importance. Nor is the alternation on opposite sides of the 

 body of the mesodermal somites and structures primarily re- 

 lated to them, such as the spinal nerves and gonads. The 

 asymmetry which is unique in Amphioxus concerns the for- 

 mation of the mouth, gill slits, and other organs connected with 

 the anterior end of the pharynx and with the oral hood. One 

 of the remarkable facts in this connection is that the asymmetry 

 is not indicated in early development and is wholly corrected 

 later in larval life, so that it is w r holly limited to a comparatively 

 brief period during larval development. It is obviously a 

 purely secondary or coenogenetic character, and must be ex- 

 plained as in some way adaptive either to present or past 

 conditions of development. We may mention but a single 

 explanatory hypothesis that of Willey. 



The starting point of this hypothesis is the assumption of a 

 primitively median dorsal position of the mouth, for which there 

 is some evidence. The second consideration is the extension of 

 the notochord forward to the tip of the snout, far in advance 

 of the central nervous system and enteron. So far as is known, 

 Amphioxus is the only form in which the chorda extends in 

 this way in all Craniates it stops in the region of the mid- 

 brain. This anterior extension of the chorda, which is ob- 

 viously an adaptation to the burrowing habit assumed at the 

 close of the larval period, occurs very early in development 

 and necessarily prevents the mouth from appearing in the mid- 

 dorsal region. The net result of this is the shifting of the 

 formative centers of the mouth, and associated structures of 

 the oral hood, to one side, actually the left, and the correlated 

 shifting of the formative centers of other pharyngeal organs. 

 The whole rotation occurs in a counterclockwise direction, 

 throwing structures of the proper left side over to the median 

 line, or even to the right side, structures of the median line 

 up on the right side, and structures of the right side proper, 

 still higher up on the same side. The region of extreme torsion 

 is around the mouth so that posteriorly the amount of dis- 

 placement becomes less and less and in the posterior part of 



