CHAPTER XIX 



THE CCELOM AND MESODERM 



In Amphioxus and all Craniates the most dorsal mesoderm is 

 segmented into somites. These each contain a portion of 

 coelomic cavity called myoccel, which persists in Amphioxus, 

 but becomes obliterated in higher forms. The median wall 

 of the myocoel is thickened and produces the myotome : a 

 plate of muscle with striated fibres, innervated by somatic 

 efferent fibres (voluntary) through the ventral nerve-roots. 

 The outer layer of coelomic epithelium lateral to the myocoel 

 gives rise to the dermatome or cutis-layer, beneath the skin. 

 On the median side, the myotome also produces the sclerotome. 

 In Amphioxus this is in the form of a hollow outgrowth, but 

 in higher forms it is composed of mesenchyme. It gives rise 

 in Craniates to the basidorsal and basiventral elements which 

 go to make up the vertebral column. 



The dorsal segmented portion of the mesoderm is known 

 as the vertebral plate. The more ventral portion of the 

 mesoderm arises segmentally in Amphioxus, each segment 

 separated from the ones in front and behind by septa. These 

 septa, however, break down, and the ventral coelomic cavity or 

 splanchnocoel is continuous from end to end of the animal. 

 This condition arises from the first in the Craniates, where the 

 mesoderm in this region, known as the lateral plate, is not 

 segmented. The outer wall of the splanchnocoel becomes 

 applied to the body- wall, and the inner wall covers the gut- 

 wall. The separation between right and left splanchnocoel 

 usually breaks down ventrally, but persists dorsally as the 

 mesentery which suspends the gut. The muscles which the 

 coelomic epithelium of the splanchnocoel produces are smooth, 



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