THE CEPHALOCORDA 199 



space, and at the same time the upper parts of the ccelomic 

 pouches containing the rudiments of the myotomes have 

 become separated off from the fused lower portions, and 

 constitute dorsal sections of the ccelom, known as myocoeles. 

 The ventral space lying below and to the sides of the gut is 

 distinguished as splanchnoccele. The partitions between the 

 splanchnocoelic sections of successive somites break down, so 

 that the splanchnocoele becomes a continuous cavity from end 

 to end of the gut. But the partitions between the myoccelic 

 sections persist and form the septa between the myotomes of 

 the adult. The figure also shows a short ingrowth or diver- 

 ticulum of the Tnyoccele below the muscle-plate at the level 

 of the lower edge of the notochord. In fig. 49, F, we see that 

 this diverticulum has grown enormously, and has pushed its 

 way upwards, separating the muscle-plate from the notochord 

 and nerve-cord above it. This diverticulum is known as 

 the sclerotome. It may further be seen in fig. 49, -F, that the 

 myoccele has pushed its way downwards, so as to separate the 

 somatopleuric wall of the splanchnocoele from the epiblast. 



We may now consider the fate of the walls of these various 

 subdivisions and prolongations of the original ccelomic 

 pouches. 



The interior wall of the sclerotome gives rise to the con- 

 nective tissue sheath of the notochord and nerve chord. The 

 outer wall of the sclerotome gives rise to the inner connective 

 tissue sheath or fascia of the muscles of the myotomes. 



The cells of the muscle-plates are converted into the muscle 

 fibres of the myotomes. 



The outer or somatic walls of the myocoeles are converted 

 into the deeper layer or cutis of the integument. It is said 

 that the dorsal and ventral fin-spaces are also formed from 

 the myocoele. 



The inner wall of the splanchnocoele bepomes the peritoneal 

 investment of the gut, the outer wall the peritoneal lining of 

 the perivisceral coelom. The splanchnocoele itself is greatly 

 reduced in size on the formation of the atrial chamber, and 

 in the pharyngeal region is subdivided into the dorsal and 

 sub-endostylar coelomic spaces and the canals of the primary 

 gill-bars on the formation of the gill-slits. The splanchnocoele, 

 therefore, persists as the body-cavity of the adult, but the 

 myoccele and cavity of the sclerotome are obliterated, and 



