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 
Nn SOR 
B sais So 
Fic. 88.—Ventral aspect of three larvae of Amphioxus, showing the metapleural folds 
and the formation of the atrium. «ap, Atriopore ; 4, gill-slits; /f and 7/, left and’ 
right metapleural folds ; m, mouth ; w, pre-oral pit. (From Korschelt and Heider, 
after Lankester 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-shts 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 
