148 ESSENTIALS OF ZOOLOGY 



understood. The body cavity extends from near the front 

 of the enteron backwards, and the peritoneum of which it 

 consists is resolved into a visceral layer around the endoderm, 

 the splanchnopleure, and a parietal layer next the ectoderm, 

 the somatopleure. 



The inner wall of the myotome is expanded to form the 

 muscle fibres, and the outer wall is applied to the ectoderm to 

 form the connective tissue of the skin, or dermis. During 

 this formation the myocoel extends downwards outside the 

 splanchnocoel so as completely to furnish the body with the 

 lateral series of myotomes and the dermis. At the side of 

 the notochord each gives out an outgrowth, the sclerotome, 



Nerve cord 



Myocoel 



Notochord 



Splanchnocoel 



FIG. 72. Amphioxus. Transverse sections illustrating the division of the 

 coelomio segment into myocoel and splanchnocoel. After Hatschek. 



which grows upwards between the notochord and the muscle 

 layer of the myocoel and furnishes the sheaths of the noto- 

 chord and the nerve cord and the myotome connective tissue 

 (fig. 64). The myocoel not only provides the muscles but the 

 connective- tissue septa for their origin and insertion. Dorsally 

 a similar outgrowth of the myocoel supplies the connective- 

 tissue supports of the dorsal fin. Later the lower edges of 

 the myocoel give rise to the supports of the ventral fin. 



The part played by the myocoel does not end with this. 

 Germ cells appear in the wall of the myocoel just ventral to 

 the sclerotome. They are formed close to the anterior septum, 

 and as they grow they project into the cavity in front in 

 each case. They are finally separated as special diverticula 

 containing the gonads. 



Before all this has happened with reference to the myocoel 

 the dorsal part of the embryo is rotated forwards, and in asso- 

 ciation with the rotation a proboscis is formed. The anterior 



