DEVELOPMENT OF THE LARGE INTESTINE 215 



In briefly summarizing the development of villi, it may be said that 

 the general tendency throughout the whole of the small intestine is 

 for villi to develop as separate invaginations of the epithelium. Owing 

 however, to the occurrence of transitory structures (vacuoles, diverti- 

 cula, and folds) their development is manifested differently in differ- 

 ent parts of the intestine. 



Although in the large intestine formation of villi is preceded 

 by distinct longitudinal folds, it does not seem probable to me, 

 after a study of my models, that there is a mechanical segmen- 

 tation of the longitudinal folds. It must be remembered that 

 the epithelial tube is ever growing by an increase in the number 

 of its cells, and by the enlargement and duplication of its parts. 

 Because of this active growth one would expect, therefore, villi 

 to form by an active direct process, rather than by an indirect 

 one. It seems more probable to me, that small knob-like eleva- 

 tions are developed along the tops of the folds, and that these 

 knobs form into villi. If such were the case, the picture pre- 

 sented would always be one w^hich would appear like a segmen- 

 tation of folds. As the villi grew taller and taller, the segment- 

 ing fissures would appear to be sinking in deeper and deeper. 

 It is important in this connection to note that the original folds 

 are considerably smaller than the villi, a fact which is in favor 

 of the view just proposed. Regarding the further development 

 of villi, it may be said with certainty, that they arise separately 

 in between those already present. 



The earliest glands develop as small knob-like grow^ths of the 

 epithelium into the mesenchyma. As has been pointed out 

 before, the additional glands are probably formed by a longi- 

 tudinal splitting of those already present. 



The villi reach their maximum height in embryos of 110 mm. 

 to 140 mm. in length. From this stage on they gradually be- 

 come smaller and smaller. A fusion of adjacent villi together 

 at their bases, which gradually extends tow^ards their apices and 

 lengthens the intestinal glands, has been described by Koelliker 

 ('61) and Schultze ('99) as the manner in which they disappear. 

 Brand ('77) describes the connective tissue about the glands 

 as the active factor in their disappearance. He believes that 



