164 



ALIMENTARY CANAL 



established. Cases of congenital stenosis of the gut just beyond the pylorus are possibly to be 

 explained by the persistence of the early occlusion. 



The eeecum is at first a uniform conical diverticulum. By the end of the 

 first month it shows a smaller apical and a larger basal section. The former 

 becomes the vermiform appendix, the latter the caecum. The caecum is at first 

 conical (fig. 206), with the appendix passing off from its apex; but later, owing to 

 the unequal dilatation of its anterior and right walls, the appendix comes to be 

 attached to its posterior and left aspect. 



Formation of the entodermic cloaca and anus. The hind-gut ends 

 during the third week in a dilated chamber, which also receives the openings of 

 the Wolffian ducts (fig. 203 A, B.). This chamber, which is called the cloaca 

 entodermica, is closed on its ventral aspect by a membrane derived from the 

 persistent part of the primitive streak. This cloacal membrane consists of 

 ectoderm and entoderm, which are here in contact with one another without the 

 intervention of mesoderm, and it forms a septum between the cloaca and a 



body-wall 



urogenital sinus 

 rectum 



FIG. 207. PELVIS OF A HUMAN EMBRYO OF 14 MM. (FIVE WEEKS). (After Keibel, from 

 Kollmann's Entwickelungsgeschichte.) 



+ bladder ; * septum uro-rectale. 



shallow surface depression (urinogenital fossa). The chamber is then divided by 

 a septum into a dorsal and a ventral passage, becoming the rectum and urogenital 

 sinus respectively (figs. 204, 205, 206), and these come to open on the surface by 

 the absorption of the membrane. The opening into the alimentary canal becomes 

 the anus. It does not correspond with the posterior extremity of the primitive 

 hind-gut, for at first there is a postanal cul-de-sac the tail-gut, which later 

 becomes reduced and obliterated. The anal opening is completed during the third 

 month. The process by which it is formed will be dealt with later. 



FORMATION OF THE GLANDS OF THE ALIMENTARY CANAL. 



Under this head may be included not only those organs which are ordinarily so 

 termed, but also the lungs, thymus, thyroid, and pituitary body, since the early 

 development of these organs resembles that of the true secreting glands. 



All the organs above enumerated are formed as epithelial involutions, either 

 solid at first and afterwards becoming hollowed out, or hollow from the first. As 



