48 ON EMBRYOLOGY AS AN AID TO ANATOMY 



to which, when in difficulty, he has of late years 

 turned so often and so rarely in vain ; and I wish 

 now to ask your attention for a short time while I 

 endeavour to make clear to you the evidence which 

 embryology offers on this point. 1 must again ask 

 your indulgence if I have to refer to matters 

 many of which are perhaps painfully familiar to 

 you, while others may appear at first sight to have 

 nothing whatever to do with the matter in hand. 

 This evidence applies only at present to Elasmo- 

 branch fishes, but the conditions of the problem are 

 absolutely the same as in man. 



You all know that there is in the trunk a space 

 between the body walls and the alimentary canal 

 called the pleuro-peritoneal cavity or body cavity, 

 or, still better, the ccelom, this space or cavity 

 being absent in the head and neck. That is, if 

 you were to push a sharp instrument through the 

 walls of the abdomen into the intestine the instru- 

 ment would not pass through solid tissue along its 

 whole course but would traverse a cavity the 

 ccelom before reaching the intestine. While if 

 you performed a similar operation in the neck the 

 instrument would pass through no cavity but would 

 penetrate solid tissue all the way until the ceso- 

 phagus was reached. Now the whole body is formed 

 from three cellular layers or strata the epiblast, 

 mesoblast, and hypoblast and if we make a 

 diagrammatic section through the body the epiblast 

 will form the outermost layer or skin ; the hypoblast 

 will form the lining of the alimentary canal ; all the 

 rest will be mesoblast. The body cavity or ccelom 



