40 



PHYLUM CEPHALOCHORDA. 



the myotome ; it retains its original segmentation and the septa 

 separating the successive somites here persist as the myosepta 

 (intermuscular septa). In the ventral portion (comparable to 

 the lateral mesoblastic plate of the Vertebrata) the transverse 

 septa, formed by the adjacent walls of the successive somites of 

 the same side, break down and the cavities of the somite become 

 continuous to form the splanchnocoele or body-cavity of the 

 adult. The cells of the inner wall of the myotome become con- 

 verted into muscles, and consti- 

 tute the lateral muscles of the 

 adult, while the outer wall 

 which is applied to the ectoderm 



Fio. 26. — Transverse section through 

 the middle of the body of an Amphi- 

 oxvs embryo witli 11 somites. On the 

 right side the section has managed to 

 cut two somites (from Korschelt and 

 Heider). oA; ectoderm ; c?i notochord ; 

 dh enteron ; ik endoderm ; Ih coelom ; 

 mk' somatic, mk" splanchnic layer 

 of mesoderm ; n nerve tube ; us meso- 

 blastic somite. 



Fio. 27. — Transverse section of 

 the middle of the body of an 

 Amphioxus larva with five 

 gill-slits, diagrammatic (from 

 Korschelt and Heider). 1 

 outer wall of myotome (cutis 

 layer) ; 2 inner or muscular 

 wall of myotome ; 3 com- 

 mencement of the sclerotome ; 

 4 septum between myocoele 

 and splanchnocoele : 5 somatic 

 mesoderm ; 6 splanchnic 

 mesoderm ; / myocoele ; // 

 splanchnocoele. The sub-in- 

 testinal vein is shown in the 

 splanchnic mesoderm. 



remains thin (Fig. 27). The sclerotome is an outgrowth 

 from the ventral and inner wall of the myocoele (Fig. 27). It 

 acquires a considerable development extending dorsalwards 

 between the muscles and the notochord and spinal chord (Fig. 28). 

 Its inner wall gives rise to the sheath of the notochord and of the 

 nerve cord, while its outer wall forms the so-called fascia-layer, 

 or internal sheath (Fig. 29). 



The dorsal part of the myocoele is said to give rise to the 

 dorsal fin canal ; and a ventral extension of the same space to 

 the ventral fin canal. The myocoele appears to abort in the 

 adult, but the sclerocoele probably in part persists as the lymph 

 spaces on the internal sides of the lateral muscles. 



