THE ALIMENTARY CAXAL. 



133 



H>2 



103 



FIGS. 102 AND 103. Early form of the alimen- 

 tary canal. (From Kolliker'afterBischoff.) In 102 

 a front view, and in 103 an antero-posterior section 

 are represented, a. Four pharyngeal or visceral 

 plates. 6. The pharynx, c, c. "The commencing 

 lungs, d. The stomach. /, /. The diverticula 

 connected with the formation of the liver, g. 

 The yolk-sac into which the middle intestinal 

 groove opens, h. The postrior part of the 

 intestine. 



are developed the pharynx, (esophagus, stomach, and duodenum ; from the hind- 

 gut, a part of the rectum : and from the 

 middle division, the rest of the intestinal 

 tube (Figs. 102 and 103). The changes 

 which take place in the fore-gut are as fol- 

 lows : The middle portion becomes dilated 

 to form the stomach, and undergoes a ver- 

 tical rotation to the right, so that the pos- 

 terior border, by which it is attached to the 

 vertebral column by a mesenterv, is now 

 directed to the left, and the anterior border 

 to the right. At this time it is straight, but 

 it soon undergoes a lateral curve or bend to 

 the right at its upper end. It thus assumes 

 an oblique direction, and the left border 

 (originally the posterior or attached border) 

 becomes inferior, and forms the great cur- 

 vature. The mesentery by which it was 

 attached forms the great omentum. The 

 portion of the fore-gut above this dilatation 

 remains straight, forming the pharynx and 



-hagus. while the part below the dilated 

 stomach forms the duodenum, and in con- 

 nection with this the liver and pancreas are 

 developed. 



The hind-gut is also a closed tube, and from it the middle third of the rectum 

 is developed, as well as the allantois (page 113), which will be again referred to in 

 connection with the development of the bladder. 



The mid-yut is at first an open cavity freely communicating with the umbilical 

 vesicle. As the body-walls grow, this communication contracts very materially, 

 though it still exists to a certain extent, and the open cavity becomes converted 

 into a straight tube, still open where it communicates with the umbilical vesicle. 

 This tube grows rapidly in length, and presents a primitive curve or loop down- 

 ward and forward, and, in consequence of its growth exceeding that of the walls 

 of the body-cavity, a portion of the loop protrudes into the stalk of the umbilical 

 vesicle. At a subsequent period, however, the walls of the abdomen grow more 

 rapidly than the intestine, which again recedes into the body-cavity. At a short 

 distance below the most prominent point of this loop a diverticulum arises, which 

 marks the separation between the large and small intestine. The lower part of this 

 diverticulum forms the vermiform appendix : the proximal part, by its continued 

 growth, constitutes the ca?curn. After this the anterior or upper part of the gut, 

 corresponding to the small intestine, rapidly increases in length, and about the 

 eighth week becomes convoluted. The lower or posterior part, corresponding to the 

 large intestine, is at first less in calibre than the upper part, and lies wholly to the 

 left side of the convolutions of the small intestine ; but later on the curve of the 

 large intestine begins to form, and the first part (ascending colon) slowly crosses 

 over to the right side, first lying in the middle line, just below the liver. It is not 

 until the sixth month that the caecum descends into the right iliac fossa, and so 

 drags the ascending colon into its normal position in the right flank. 



The peritoneal cavity is the space left between the visceral and parietal layers 

 of the mesoblast. and the serous membrane is developed from these structures. The 

 mesenteries are formed from mesoblastic tissue extending between the vertebrae and 

 the gut which develops the vascular and connective-tissue elements of these parts. 



The buccal cavity is formed by an involution of the external layers of the 

 blastodermic membrane, which passes inward and meets the pharynx, or upper 

 part of the fore-gut. The two cavities are, however, at first completely separated 

 from each other by all the layers of the blastoderm ; but at an early period of 



