REPRODUCTION 99 



blood vessels richly interspersed, and the whole complex ensheathed by 

 connective tissue and thereby dehmited from adjacent tissues of the 

 body-wall. 



The hypomere mesoderm, later backed up by a layer of connective 

 tissue, becomes the definitive peritoneum. Its somatic layer completely 

 lines the body-wall. Its visceral layer covers the coelomic surfaces of the 

 digestive tube, of such derivatives of the tube as the liver and pancreas, 

 and of all other organs which occupy the coelom. As the originally 

 bilateral coelomic spaces expand to accommodate the developing viscera, 

 the right and left visceral (splanchnic) layers of the hypomere meet one 

 another in the median plane at all regions not occupied by median organs 

 (Fig. 63). Here the two layers coalesce and, later reinforced by formation 

 of connective tissue between them, become the system of membranes or 

 mesenteries which connect and support the viscera. These median mes- 

 enteries are usually much more extensive in the embryo than in the adult. 

 In later stages of development they undergo considerable reduction, espe- 

 cially those between the digestive tube and the ventral body wall (Fig. 67). 



The mesenteries not only support the major visceral organs but they 

 are important as bridges across which nerves and blood vessels pass from 

 the body wall to organs which are suspended within the coelom, or they 

 may support ducts which traverse coelomic space in passing from one 

 organ to another — the bile duct, for example. In the earHer embryo 

 the establishing of necessary relations of nerves, blood vessels and ducts 

 to their appropriate organs is facilitated by the extensive system of 

 mesenteric bridges. After proper connexions have been made, consider- 

 able regions of the mesenteries may be obliterated. A blood vessel which 

 may appear to extend across coelomic space freely and without support is 

 always encased in peritoneum which was left surrounding the vessel after 

 the original supporting mesentery degenerated. The mesenteries of the 

 early embryo to some extent serve as scaffolding, necessary during the 

 period of construction, but removed after the work is finished. 



Many complications arise in consequence of the secondary elongation, 

 bending and twisting of the digestive tube. For example, at a given 

 transverse section of the animal, the digestive tube may be cut two or more 

 times (Figs. 67 and 68), one section of it lying above or alongside another 

 and mesentery extending between adjacent regions. Lateral branches of 

 the median mesentery may be developed in relation to lateral organs such 

 as gonads. 



Consideration of the relations of the peritoneum to the viscera makes 

 it clear (Fig. 73) that no organ can be said to lie in the coelom except 

 as the peritoneum investing that organ is regarded as a part of the organ. 

 In strict morphological sense, no organ lies in the coelom. The essential 

 organ — that is, the structure constituted of tissues specialized for a 



