William A. Hilton 461 



1. Fixation by two or more methods. 



2. Naked eye examination, Avith isolation of villi to be studied under 

 a lens or microscope. 



3. Cutting free-hand sections if possible. 



4. Mounting mucosa up after staining, if the muscular walls were 

 thin enough. 



5. Cutting serial sections from tissues imbedded in paraffine or 

 collodion. 



For the work on embryological development of folds and villi the 

 usual fixing and staining fluids were employed. 



Character and Occurrence of Folds and Villi. 



Mucosal elevations of the intestines may be considered under four 

 heads: first, such as the typhlosole and spiral valve; second, valvulse 

 conniventes; third, simple more or less longitudinal mucosal folds; 

 fourth, villi. 



The first and second divisions hardly come under the scope of this 

 discussion, but will be briefly considered. 



It may be said that the typhlosole of lamprey is a simple or partially 

 developed spiral valve. In Elasmobranchs, a high development of this 

 organ is reached; here the spiral valves vary much in complexity, but 

 are traceable directly or indirectly, with the help of embryology, to 

 a simple type such as the typhlosole of lamprey. The Ganoidea appar- 

 ently have more or less distinct remnants of spiral valves, while this 

 peculiar type of intestinal folds is found to have reached a high develop- 

 ment in the Dipnoi and Holocephali as well as the Elasmobranchii. 



Kuckert, 96, described the development of the spiral valve of Pris- 

 tiurus from spiral foldings of the mucosa; but it seems to me that the 

 spiral valve is like a folding of the whole thickness of the intestine 

 rather than of the mucosa and submucosa alone, and in the course of 

 time the infoldings may have lost much of the original character of the 

 infolded intestinal wall. I shall not attempt to explain just how or 

 when this infolding took place, but it seems probable that it began 

 before or with the formation of a simple spiral fold like the typhlosole. 

 This infolding may be considered as similar to several coils of intestine 

 which have come into very close contact; no essentially new structure 

 ' is formed, simply a modified intestinal tube. The folding of the spiral 

 is probably as much for retarding the progress of the food as for 

 increased absorption area, because the absorbing area is not much 

 greater than that of a coiled, small caliber intestinal tract; for where 



