192 



inner or splanchnic wall, the muscles of the body wall are 

 formed, and from its outer or somatic wall arises the cutis 

 or basis of the skin. 



The inner wall at its lower end shows that proliferation 

 (characteristic of mesoderm) which results in a mass of 

 discrete cells or mesenchyme. (See page 16, Part I.) This 

 outgrowth (the sclerotome) extends inwards then upwards 

 between the myotome and the notochord, and forms the 

 connective tissue which sheathes the notochord, spinal 

 cord, and other parts. The myotomes remain separate ; 

 but the ventral portions of the somites lose (by absorption) 

 their septal walls, and form a continuous lateral plate, 

 the cavity of which is the splanchnocoele, the future coelome 

 or body cavity of the adult (and enterocoelic, therefore, 

 in origin). The two lateral plates meet together ventrally, 

 their cavities become continuous and thus form the main 

 body cavity or peritoneal cavity. 



Additional primary gill-slits are formed ventrally and 

 shifted on to the right side ; and above them eight secondary 

 gill-slits develop. Later, the first and the last five primary 

 slits disappear and the remaining eight are shunted on to 

 the left side ; there is then an equal number of eight gill-slits 

 on each side. The number increases, and the slits become 

 vertically elongated. 



On the under surface of the body two longitudinal folds 

 of skin (metapleural folds) appear behind the gill-slits and 

 grow forward above them. From each fold an inner shelf 

 grows out ; the two coalesce, and thus a canal, the atrium, 

 is formed. It enlarges, pushes in the ventral body- wall, 

 compresses the coelome and extends right round each 

 side of the gut, as far back as the middle of the intestine ; 

 there it opens externally by the atriopore. Therefore the 

 gill-slits no longer open directly to the outside, but into the 

 atrial chamber. 



The neurenteric canal is closed. The endostyle is formed. 

 The mouth is shifted to a median position ; it is reduced 

 to a small opening, its margin becomes the velum, and 

 two folds grow out in front and form a vestibule, the 

 oral hood. The gonads appear as thickenings on the myocoele 

 wall, at the lower ends of the myotomes. The portion of 

 myocoele containing the gonad is constricted off later. 



The larval period extends to about three months. Change 

 of habitat marks the transition to <he adult state ; pelagic 

 life is abandoned, and the young Amphioxus takes to the 

 sand. A feature of the development is the asymmetry of 

 the larva, as seen, for example, in the original position of 

 the mouth and the primary gill-slits. 



