1890.] Development of the Atrial Chamber of Amphioxus. S7 



They are at first extremely small, and the atrial space floored in in 

 a mere canal. Later the width of the atrial space increases greatly, 

 and the sub-atrial folds consequently widen 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.r.). 



The atrial groove becomes floored in first in the region of the 

 atriopore. The growth of the sub-atrial folds extends gradually 

 forwards, and the closure proceeds along one side (the right) of the 

 pharynx. The whole atrium thus formed is a very small tube-like 

 space. The closure by means of the small horizontal sub-atrial out- 

 growths in the region of the large gill-slits is somewhat difficult to 

 explain. The small left metapleur actually moves in course of 

 growth from the mid-line, and rises on to the right side somewhat. 

 At the same time the much larger right metaplenr is deepened, and 

 overhangs the slits. Then the little horizontal junction is effected, 

 and we get actually a nearly tubular atrium receiving the openings 

 of successive gill slits. With subsequent growth the narrow atrial 



FIG. 8. 



dim 



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

 latter has begun to encroach on the ccelom (*p). 

 Letters as in figs. 6 and 7. 



