48 OUTLINES OF CHOKDATE DEVELOPMENT 



ridge or shelf appears, just below the level of the body wall. 

 These are the subatrial ridges (Fig. 18, A); they grow toward 

 one another and fuse across the mid-line, enclosing between 

 themselves and the ventral body wall a small space lined with 

 ectoderm, which is the beginning of the atrial cavity (Fig. 18, B) . 

 The formation and fusion of the subatrial ridges occur first pos- 



sa 



FIG. 18. Diagrams of transverse sections through Amphioxus larvae, showing 

 the formation of the atrial cavity. After Lankester and Willey. A. Section 

 through larva with eleven or twelve primary gill slits, showing subatrial ridges on 

 metapleural folds. B. Section through slightly older larva (Fig. 17, 5), showing 

 first fusion of subatrial ridges to form the rudiment of the atrial cavity. C. 

 Section through advanced larva showing enlargement of atrial cavity and the 

 method of its encroachment upon the coelom. a, Atrial cavity; ao, dorsal aorta; 

 c, ccelom (splanchnoccel) ; ch, notochord;/, dorsal fin cavity; i, intestine; ra, meta- 

 pleural fold containing ccelomic space; mt, myotome; my, myoccel; n, nerve cord; 

 sa, subatrial ridges; sc, sclerotome; v, subintestinal vein. 



terior to the gill-slit region, and proceed thence forward, and by 

 the time the secondary gill-slit series is completed the bran- 

 chial region is completely covered. Anteriorly the subatrial 

 ridges merge with the body wall closing the cavity at this end, 

 but posteriorly the atrial cavity remains open to the outside as 



