THE DEVELOPMENT OF VERTEBRATA. 



227 



Then the pouches open to the exterior and become gill- 

 slits. The slits increase in number by new ones being 

 formed in turn behind (as is the case with the other meta- 

 meric structures). 



The metapleural folds and atrium are formed when there 

 are only a few gill-slits, and the diagram (fig. 116) indi- 



A B 



Fig. 116. ORIGIN OF ATRIUM AND GILL-SLITS IN AMPHIOXUS. 



(After Lankester and Willey, altered.) Epiblast black, mesoblast dotted, hypoblast 

 lined. In B, the barbed arrow shows the formation of a gill-slit, the uribarbed 

 arrows show the extension of the atrium. 



cates their mode of origin. First the metapleural folds are 

 formed, as longitudinal ridges of epiblast and somatic 

 mesoblast. Then ingrowths towards the middle line arise 

 from these and meet, enclosing a space lined by epiblast 

 this is the atrium. It is at first small but soon grows, 

 pushing its way dorsally on either side of the alimentary 

 canal, as shown by the arrows in fig. 116, B. The mode of 

 origin farther forwards, where the slits are already formed, 

 is the same in principle; but the metapleural folds first 

 appear above the gill-slits, and grow down to enclose them. 



