DEVELOPMENT OF ATRIAL CHAMBER OP AMPHIOXUS. 453 



a small extent. It dies out in front of the first gill-slit^ where 

 it bends towards the middle line. 



The atrium is formed by a small horizontal growth (s.a.7'. in 

 Fig. 6), which starts from the inner face of each metapleur and 

 floors in the deeper half of the groove or area between the two 

 metapleura (Fig. 7, at.). 



These horizontal growths may be called the sub-atrial 

 folds. 



They are at first extremely small, and the atrial space 

 floored in is a mere canal. Later the width of the atrial space 

 increases greatly, and the sub-atrial folds consequently widen 



Fig. 8. 



(Jim — 



VlC'tt 



Transverse section through an advanced larva with fully-closed atrium. Tiie 

 latter has begun to encroach on the ccelom (splanchnoccel) (*/>.). Letters 

 as in Figs. G and 7. 



also, becoming that pleated expansible floor of the atrial 

 chamber, with its transverse muscular layer, which all observers 

 of Amphioxus know so well (Fig. 9, s.a.7\). 



