I04 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



(mesenteries, in the broader sense of the word) which connect 

 the various viscera to the walls of the body cavity. The condi- 

 tions in the abdominal region will be described first. 



Here the splanchnic la}er of the mesothelium applies itself 



to either side of the 

 walls of the alimen- 

 tary tract, it being 

 of course kept in 

 mind that mesenchy- 

 matous tissue has 

 migrated in between 

 entoderm and meso- 

 thelium in this re- 

 gion (see p. 103), 

 while above and be- 

 low the digestive 

 tract the dorsal and 

 ventral walls of the 

 hypomere press in- 

 wards towards the 

 median line, insinu- 

 ating themselves 

 dorsally between the 

 alimentary canal and 

 the notochord, ven- 

 trally between the 

 entoderm and ecto- 

 derm. As a result 

 there is formed a 

 double partition be- 



FlG. 113. Diagrammatic section of vertebrate 

 through abdominal region, a, dorsal aorta ; c, ccelom; 

 g^ gonad; gl^ glomerulus; /, alimentary canal; /, 

 liver; ;«, mesentery ; OT!i, muscular layer of myotome ; 

 /;/)', myocoele ; K, nephrostomy; ;/<?, neural arch; ?/r, 

 notochord ; o, • omentum ; s, spinal cord ; so, somatic 

 layer of peritoneum ; J/, splanchnic layer of peri- 

 toneum ; /, nephridial tubule; vm, ventral mesentery ; 

 ti', Wolffian duct. 



tween the metacoeles 

 of the two sides both above and below the intestine, with a 

 small amount of mesenchymatous tissue between the two epi- 

 thelial walls. These partitions, which thus come to support 

 the alimentary canal (Fig. 1 1 3), are the dorsal and ventral 

 mesenteries. 



The ventral mesentery is never perfect throughout the 

 abdominal cavity. In the posterior portion the partition walls 



