CHAP, xm ENTERIC CANAL 597 



of course communicates with the mesenteron ; but as the 

 embryo is gradually folded off from the yolk-sac, the 

 mesenteron becomes tubular along its whole length, 

 and eventually the stalk of the yolk-sac becomes solid, 

 at hatching (or birth) its point of communication with 

 the body being marked by the navel (umbilicus). The 

 ectodermic stomoct&um (p. 204), which gives rise to the 

 mouth-cavity and also to the greater part of the pituitary 

 body, comes to communicate with the mesenteron 

 anteriorly, and the ectodermic proctodceum opens into 

 it posteriorly. The muscular and peritoneal layers of 

 the canal are formed, it will be remembered, from the 

 mesoderm. 



The first traces of the liver and pancreas are seen 

 as simple endodermic offshoots of the mesenteron, 

 which gradually become branched in a complicated 

 manner, the numerous lobules being more or less closely 

 connected together by mesoderm. The gill-pouches arise 

 as paired outgrowths of the endoderm lining the pharynx, 

 which come into contact with the ectoderm, the latter 

 becoming perforated to form the external branchial 

 apertures. Four gill-clefts appear in the embyro of 

 reptiles, birds, and mammals animals in which gills are 

 never developed (Figs. 158 and 160) ; but they early 

 disappear with the exception of the first cleft, corre- 

 sponding with the spiracle of the dogfish, which gives 

 rise in all Vertebrates above fishes to the tympano- 

 eustachian passage (p. 449) : the branchial skeleton, as 

 we have seen, undergoes a corresponding reduction or 

 modification (pp. 438 and 496). In air-breathing Verte- 

 brates the lungs arise as a ventral outgrowth of the 

 pharynx, lined by endoderm and covered by mesoderm. 



The circulatory organs are developed from the 

 mesoderm, the heart arising in the visceral layer on the 



