PKIMAEY FORM OF THE ALIMENTARY CANAL, 



■To 



sages communicating witli it, is at first extremely thin and simple, and 

 is composed of flat cells ; but as soon as this layer comes to form a part 

 of the. inflected alimentary tube, its character is completely altered, its 

 cells become cylindrical, and it attains a great proportional thickness, 

 which it preserves for a considerable time. 



Fig. 576. 



Fig. o7i3. — Early form 

 OF THE Alimentary' 

 Canal (from Kolliker 

 after Bischotf). 



In A a front view, 

 and in B an antero-pos- 

 terior section are repre- 

 sented. 



a, four pliaryngeal or 

 visceral plates ; h, the 

 phai-ynx ; c, c, the com- 

 mencing lungs ; d, the 

 stomach ; /, /, the di- 

 verticula connected with 

 the formation of the liver ; 

 g, the yolk-sac into which 

 the middle intestinal 

 gi'oove opens ; h, the 

 posterior part of the in- 

 testine. 



The outer surface of the inflected hypoblast of the alimentary tube is 

 more or less in contact with the splanchno-pleure division of the mesoblast. 

 In the head no marked separation of the splanchnopleure and somato- 

 pleure divisions of the mesoblast takes place, but the elements of the former 

 are no doubt combined with the hypoblast in the walls of the pharynx, 

 and the formation of the tympano-eustachian and following pharyngeal 

 clefts is therefore due to the perforation of both epiblastic and hypo- 

 blastic layers with intervening mesoblastic tissue, just as occurs in the 

 formation of the opening of the fauces. But in the thorax and 

 abdomen, the primitive alimentary canal is brought into relation with 

 the pleuro-peritoneal cavity, and receives in various parts a serous 

 investment from the lining membrane which becomes developed in 

 that space. In the thorax the right and left cavities remain distinct 

 as the two pleura, while a central portion is separated for the forma- 

 tion of the pericardium, and thus the gullet, as well as the lungs, is 

 brought into relation with the pleura, and receives partial covering 

 from them. The formation of the diaphragm, which does not at first 

 exist, and which grows down from the vertebral column as a partition 

 between the thorax and abdomen, leads to the ultimate separation of 

 the peritoneum from the pleurte. Some examples of diaphragmatic hernia 

 may be considered as arising from the persistence of the original 

 connection between the two cavities. In the abdomen, also, the right 

 and left peritoneal cavities are at first distinct, but when the intestine 

 assumes a tubular form, the right and left cavities are thrown into one 

 across the middle plane of the body. 



As the development of the alimentary canal proceeds, the middle 

 open part shortens, more and more of it being converted into the 

 tubular intestiue, and at last, as before explained, there remains only 



