THE BODY CAVITY. 513 



as the dorsal mesentery, while ventrally they are in most instances 



3 



v 



FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA LABYRINTHICA. 



The section is taken slightly to one side of the middle Hue so as to shew the rela- 

 tion of the mesoblastic somites to the limbs. In the interior are seen the yolk 

 segments and their nuclei. 



1 16. the segments ; pr.l. proeephalie lobe ; iln. dorsal integument. 



absorbed. The transverse walls, separating the successive compart- 

 ments of the body cavity, generally become more or less perforated. 



Chordata. In the Chordata the primitive body cavity is either 

 directly formed from a pair of alimentary diverticula (Cephalochorda) 

 (fig. 3) or as a pair of spaces in the mesoblastic plates of the two 

 sides of the body (fig. 20). 



As already, explained (pp. 244- 24*9) the walls of the dorsal sec- 

 tions of the primitive body cavity soon become separated from those 

 of the ventral, and becoming segmented constitute the muscle plates, 

 while the cavity within them becomes obliterated : they are dealt 

 with in a separate chapter. The ventral part of the primitive cavity 

 alone constitutes the permanent body cavity. 



The primitive body cavity in the lower Vertebrata is at first con- 

 tinued forwards into the region of the he;ul, but on the formation of 

 the visceral clefts the cephalic section of the body cavity becomes 

 divided into a series of separate compartments. Subsequently these 

 sections of the body cavity become obliterated ; and, since their walls 

 give rise to muscles, they may probably be looked upon as equivalent 

 to the dorsal sections of the body cavity in the trunk, and will be 

 treated of in connection with the muscular system. 



As a result of its mode of origin the body cavity in the trunk is 

 at first divided into two lateral halves ; and part of the rnesoblast 

 lining it soon becomes distinguished as a special layer of epithelium, 

 known as the peritoneal epithelium, of which the part bounding 

 the outer wall forms the somatic layer, and that bounding the inner 



n. E. ii. 33 



