MESODERM AND COELOME 



561 



cavities of these, which subsequently give rise to the 

 coelome {Ih, usk), are thus at first in free communication with 

 the archenteron and are known as enterocceles ; from their 

 walls the mesoderm is derived. Subsequently the communi- 

 cations between the enteric and enterocoelic cavities become 



Fig. 144. — Kmbryo of /4 ;«//i/(7.i-M.r. 

 .^j in vertical section, slightly to the left of the middle line; B, in horizontal 

 section, ak. ectoderm ; c«. neurenteric canal ; dk^ ud. archenteron ; ik. endo- 

 derm ; ink. mesodermal folds ; n, medullary canal ; us. first coelomic pouch ; 

 ush. coelomic cavity ; V. anterior ; H. posterior end. (From Korschelt and 

 Heider, after Hatchek.) 



closed, and the paired pouches gradually extend between the 

 ectoderm and endoderm, both dorsally and ventrally (Fig. 

 143, C, D), their outer walls (parietal or somatic layer of the 

 mesoderm, mK^') being in contact with the ectoderm and 

 forming with it the somatopleure or body-wall, and their 

 inner walls (visceral or splanchnic layer of the mesoderm, 



Pract. Zool. o o 



