DEVELOPMENT OF THE RECTUM 15 



the cloaca is reached, it gradually increases in size again. This 

 lower widened portion, which is relatively short, joins the cloaca. 



Separation of the rectum and uroginital sinus has taken place 

 in an embryo of 16 mm. (H.E.C. 1322). The spindle-shaped 

 swelling is more pronounced than before, and its epithelium now 

 passes over into that of the outside skin. Just before the epi- 

 dermis is reached, however, another slight swelling is seen, smaller 

 and more flattened than the swelling above. The lumen of the 

 rectum is continuous above with that of the colon, is larger in the 

 swellings than in between them, and below opens to the exterior 

 by an extremely fine opening. In a second embryo of 16 mm. 

 (H. 133) the epithelial tube has a similar form. The lumen, 

 however, is occluded at its lower end. 



In embryos of 17 mm. (H. 58) and 19 mm. (H.E.C. 819) both 

 swellings have increased slightly in size and are more flattened 

 dorso-ventrally. Again the lumen does not open to the outside for 

 at its lower extremity can be seen a mass of cells which fills the 

 lumen. 



At 22.8 mm. (H.E.C. 871) the epithelial tube presents a form 

 as seen in figures 7 and 11. The two swellings are again present, 

 the upper of which is much larger and more conspicuous. Both 

 are again slightly flattened dorso-ventrally. A very shallow de- 

 pression can be perceived running along the ventral surface of the 

 upper swelling thus forming a small infolding of the epithelium 

 into the lumen. This fold, the first of a large number to develop 

 in the walls of the rectum, may be regarded as the earliest appear- 

 ance of a rectal column. 



Throughout all the preceding stages the epithelial tube is made 

 up of a stratified epithelium of two to four laj'^ers of cul^oidal 

 cells, without distinct cell boundaries, an epithelium quite similar 

 to that found in the whole of the digestive tract in its early stages. 

 In the region of the swellings the epithelial wall becomes corres- 

 pondingly thicker. 



Surrounding the above described epithelial tube of the rectum 

 is seen loose mesenchyma. The circular layer of the muscularis 

 was first seen in an embryo of 16 mm. It extends down only as 

 far as the constriction in between the two swellings, a point to 



