CHAPTER III 

 THE ALIMENTARY CANAL 



The alimentary canal or enteron of the Vertebrate consists of a 

 tube passing from the mouth to the anus. The wall of this tube is 

 known technically as the splanchnopleure in contradistinction to 

 the somatopleure or body-wall (Balfour). It consists of an inner 

 lining epithelium, the endoderm, ensheathed in a complex coating 

 of mesoderm — the splanchnic mesoderm — consisting of connective 

 tissue, blood-vessels, lymphatics, nerves, and coelomic or peritoneal 

 epithelium. 



As is commonly the case in other inetazoa the endodermal 

 lining is in the Vertebrate more or less encroached upon at the 

 oral and anal ends of the tube by the spreading inwards of 

 ectoderm. The parts of the tube which come thus to be lined with 

 ectoderm are known as stomodaeum and proctodaeum (Lankester, 

 1876) while the intervening region lined by endoderm is known as 

 the mesenteron. In the Vertebrata there is very slight develop- 

 ment of proctodaeum but an important section of the buccal cavity 

 is, as will be seen later, stomodaeal in its nature. 



It is also customary in. embryological writings to use the some- 

 what loose expression foregut for the anterior portion of the 

 alimentary canal (reaching back to the pylorus or to the opening 

 of the bile-duct), which in the meroblastic vertebrates becomes 

 differentiated off from the yolk-sac comparatively early in de- 

 velopment. 



A good idea of the blocking out. of the main regions of the 

 alimentary canal in one of the lower vertebrates is got by inspecting 

 sagittal sections of embryos and larvae at different stages of develop- 

 ment such as those shown in Fig. 80. From the gastrula stage (A) on 

 to the stage illustrated in Fig. 80, C, the endoderm forms a simple 

 sac with its opening posterior (anus) and with its ventral wall 

 greatly thickened owing to the fact that its cells contain the 

 main store of yolk. From the stage of Fig. 80, D onwards the 

 foregut (/.<?) becomes gradually constricted off in a tailward 

 direction from the mass of yolk, while at the opposite end of the 

 body, correlated with the outgrowth of the posterior trunk region of 

 the embryo and the backward shifting of the anus, the yolky mass 



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