42 



PHYLUM CEPHALOCHORDA. 



myocoele. They soon come to project into the cavity of the 

 somite in front, pushing the myosepta before them (Fig. 30). 



-/i 



FIG. 29. — Transverse section through the branchial region of AmpJdo'jrus showing on'the left 

 the condition of a secondary, and on the right that of a primary gill-bar, diagrammatic 

 (after Boveri and Hatschek, from Korschelt and Heider). ao aorta ; c cutis layer of 

 myocoele ; ec endostylar coelom ; / fascia layer ; fli dorsal fln canal ; ci genital sac ; gl renal 

 vessels ; J: vessel in primary bar ; kd pharynx ; Id ligamentum denticulatum ; m muscle- 

 plate ; mt transverse muscle ; n renal canal ; of metapleural lymph space ; p atrial cavity ; 

 sc dorsopharyngeal coelom ; si sub-pharyngeal vessel ; sk skeletogenous layer of sclerotome ; 

 vf lymph canals of the atrial floor. 



They lie therefore as small sacs in the preceding myocoele 

 attached to its hind wall by a pedicle (Fig«. 30 C, 31). Later, the 

 part of the myotome in which they lie becomes separated from 



