IV 



LARVAL STAGES 



135 



derm, the outer the somatic and the inner the splanchnic layer; 

 and the ventral parts of their cavities unite to form the coelom. 

 The cells of the dorsal parts become muscle fibres, and constitute 

 the myotomes internally and the connective tissue of the skin 

 externally. 



The larva (Fig. 87) is now long and narrow with many 

 segments, pointed ends, and a caudal fin. The gill-slits all 

 appear first in the mid-ventral line and then shift over to 

 the right side (Fig. 87, 1-4) : they are metamerically arranged. 

 After fourteen have been so formed a series of eight appear 



B 



FIG. 88. Ventral aspect of three larvae of Amphioxus, showing the metapleural folds 

 and the formation of the atrium, ap, Atriopore ; k, gill-slits ; If and rf, left and 

 right metapleural folds ; >, mouth ; w, pre-oral pit. (From Korschelt and Heider, 

 after Laukester and Willey.) 



dorsally to those on the right side, and then the first set, 

 originally ventral, move over to the left side, and by the sup- 

 pression of some they become equal in number and segmentally 

 arranged on the two sides of the body. This is perhaps the 

 stage at which Amphioxus shows the nearest approach to the 

 typical embryo of a higher Vertebrate. The gill-slits are here 

 seven to nine on each side, and the Vertebrate embryo has usually 

 five to seven on each side. These first gill-slits in Amphioxus 

 are later subdivided by the downgrowth of the tongue-bar from 

 the dorsal edge. 



The atrium is an ingrowth of the external space between 

 the two ventral metapleural or atrial folds (Figs. 88 and 89), 

 paired lateral ridges of the body-wall, and so is lined by ecto- 

 derm. This ingrowth is shut off from the exterior by the 



