THE DEVELOPMENT OF AMPHIOXUS 51 



the pharynx the parts appear and remain in a normal 

 position. 



Such an asymmetry is in itself obviously not advantageous 

 (adaptive) and during later development it becomes corrected 

 by a compensatory rotary growth of the pharyngeal structures 

 in the opposite direction, while the mouth and associated struc- 

 tures (Raderorgan, groove of Hatschek, etc.) move chiefly in 

 the anterior direction. 



This explanation of the origin and correction of the larval 

 asymmetry may be accepted though its necessarily hypothet- 

 ical character should be clearly recognized. 



10. The Mesodermal Somites 



At the commencement of the larval period there were four- 

 teen or fifteen pairs of somites formed, and we have seen how 

 each of these divides into a dorsal myotome, with its small 

 myoccel and thickened muscular wall, and a ventral lateral 

 plate with thin walls and large splanchnoccel, and further how 

 the median, anterior, and posterior faces of the lateral plates 

 break through, forming a continuous ccelom, walled extern- 

 ally by somatic and internally by splanchnic mesoderm (Fig. 

 19, A). By the time three primary gill slits have been formed 

 the number of somites has more than doubled, and the full 

 adult number (sixty-one in the common species of Amphioxus, 

 Branchiostoma lanceolatum) is acquired by the time the series 

 of primary gill slits is completed. All of these additional 

 somites are formed from the rapidly elongating posterior 

 region of unsegmented mesoderm, which has been carried 

 backward past the neurenteric region by the outgrowth of the 

 tail. 



During the later development of the somites their myoto- 

 mal region increases largely in vertical extent, and the nar- 

 rowed myoccel sends downward a thin-walled extension below 

 the myotome proper. This divides into two folds or out- 

 growths: one of these folds extends inward and upward, be- 

 tween the myotome or protovertebra as 'it is sometimes called, 



