340 FORMATION OF ORGANS. 



broken up successively from before backwards into somewhat 

 cubical bodies, in the centre of which a cavity soon appears. 

 The cavity in each somite is obviously bounded by four walls, 

 (i) an outer, the somatic, which is the thickest; (2) an inner, the 

 splanchnic ; and (3, 4) an anterior and posterior. The adjoining 

 anterior and posterior walls of successive somites unite together 

 to form the transverse dissepiments of the adult, which subse- 

 quently become very thin and are perforated in numerous places, 

 thus placing in communication the separate compartments 

 of the body cavity. The somites, though at first confined to a 

 small area on the ventral side, gradually extend so as to meet 

 their fellows above and below and form complete rings (fig. 157) 

 of which the splanchnic layer (sp] attaches itself to the enteric 

 wall and the somatic (so) to the epiblast. In Polygordius and 

 probably also Saccocirrus and other forms the cavities of the 

 somites of the two sides do not coalesce ; and the walls which 

 separate them constitute dorsal and ventral mesenteries. The 

 two cavities in the cephalic commissure unite dorsally, but 

 ventrally open into the first somite of the trunk. 



The mesoblastic masses of the head are probably not to be regarded as 

 forming a pair of somites equivalent to those in the trunk, but as forming 

 the mesoblastic part of the prae-oral lobe, of which so much has been said in 

 the preceding pages. Kleinenberg's observations are however of great im- 

 portance as shewing that the cephalic cavities are simply an anterior part of 

 the true body cavity. 



The splanchnic layer of the head cavity gives rise to the 

 musculature of the oesophagus. 



The somatic layer of the trunk somites becomes converted 

 into the musculature of the body wall and the external peri- 

 toneal layer of body cavity. The first part of the muscular 

 system to be definitely formed is the ventral band of longitudi- 

 nal muscles which arises on each side of the nervous system in 

 contact with the epidermis (fig. 157, m). How the circular 

 muscles become subsequently formed outside these muscles has 

 not been made out. 



The splanchnic layer of the trunk somites gives rise to the 

 muscular and connective-tissue wall of the mesenteron, and also 

 to the walls of the vascular trunks. The ventral vessel is first 

 formed (Kowalevsky) as a solid mass of cells which subsequently 



