Reproduction 



281 



Fig. 232. Diagrammatic transverse section of the body of a vertebrate showing 

 relations of organs to the peritoneum and coelom. (A) Dorsal aorta; (C) coelom; 

 (EN) endodermal epithelium of digestive tube; (G) gonad; (1) integument; (K) 

 kidney; (L) liver; (M) muscle layer of digestive tube; (MD) dorsal muscle of body- 

 wall; (MV) ventral muscle of body-wall; (NC) position of embryonic notochord; 

 (NT) neural tube (spinal cord); (PP) parietal peritoneum; (PV) visceral peri- 

 toneum; (R) rib; (VC) vertebral column. (Courtesy, Neal and Rand: "Chordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



interspersed, and the whole complex ensheathed by connective tissue 

 and thereby delimited from adjacent tissues of the body-wall. 



The hypomeric mesoderm, later backed up by a layer of con- 

 nective tissue, becomes the definitive peritoneum. Its somatic layer 

 completely lines the body-wall. Its visceral layer covers the coelomic 

 surfaces of the digestive tube and of all other organs which occupy the 

 coelom. In the median plane at all regions not occupied by median 

 organs (Fig. 228), the right and left visceral layers of the hypomere 

 meet one another to coalesce and become membranes or mesenteries 

 which connect and support the viscera. In later stages of development, 

 the mesenteries undergo considerable reduction, especially those be- 

 tween the digestive tube and the ventral body-wall (Figs. 230, 231). 

 The mesenteries not only support the viscera but provide embryonic 

 bridges across which nerves and blood-vessels pass from body-wall to 

 viscera, or ducts (e.g., the bile-duct) which traverse coelomic space 

 between one organ and another. An adult blood-vessel may appear to 



