19LS] Aliiucutary Canal of Altica Larva 301 



Iniinia. The intima is slightly thicker than it was in the 

 ileum; like the latter, it is differentiated into a ]jrimary and a 

 secondary intima, the former being very thin. 



Epithelium . At the point of union between the colon and 

 the ileum, the epithelial cells of one region closely resemble 

 those of the other. The intestine bends again caudad at the 

 junction, and a section through this region shows at one side 

 t^^pical ileum cells and at the other typical colon cells, while 

 between them are transition cells which might be assigned 

 to either region. The cells of the ileum nearly always show a 

 dift"erentiation into two areas of cytoplasm, while the cytoplasm 

 of the colon cells is only rarely so differentiated, this differ- 

 entiation usually disappearing in the intervening transitional 

 cells. There is at this point no difference in the size of the 

 cells. 



As one traces the course of the colon toward the rectum, 

 the cells of the epithelium become flatter and flatter, this 

 tendency beginning with the association of the Malpighian 

 vessels with the wall of the canal. Typically, the epithelium 

 of the colon is only about one-half as high as that of the ileum, 

 though the cells are nearly if not quite as wide. The cytoplasm 

 is usually of the same structure throughout, resembling the 

 inner layer of the ileum, but rarely it is feebly differentiated 

 into two layers as is the case in the ileum. 



The nuclei are of about the same size and shape as in the 

 preceding region, but the chromatin granules are slightly larger. 

 Here, too. the cell divisions are obscure, probably in part at 

 least due to the fixation. The epithelium may be unfolded, or 

 may be thrown into six or more prominent tooth-like folds, 

 which project into the lumen. 



A typical cross-section through the colon is shown in 

 figure 25. 



Basement membrane. The basement membrane is always 

 clearly defined. 



Circular muscles. The layer of circular muscles, which is 

 continuous with that of the ileum, is strongly developed in 

 the anterior portion of the colon, but the fibres become weaker 

 and weaker as one traces them caudad, although they do not 

 disappear completely until the end of this region. The most 

 posterior muscle is inserted on the intima of the colon, on an 



