THE MORPHOLOGY AND DEVELOPMENT OF INTESTINAL 

 FOLDS AND VILLI IN VERTEBEATES/ 



WILLIAM A. HILTON, 

 From the Laboratory of Histology and Embryology, Cornell University. 



*■ With 2 Tables and 87 Figures. 



For many years confused ideas have been prevalent concerning the 

 form and occurrence of mucosal folds and villi in the different divisions 

 of vertebrates, and even yet it is not entirely clear just where folds 

 cease and villi begin. There are great variations in occurrence even 

 in closely related forms, but much of this confusion may be due to 

 the various methods of study which have been employed. Compara- 

 tively few investigators have made an extended study of the folds and 

 villi in the different classes, although quite a number have made careful 

 investigations of a single vertebrate class, and much work has been 

 done upon the folds and villi of different species when the general 

 digestive system of a typical form was described. 



The phylogenetic study of an organ often reveals many perplexing 

 structures which may render conclusions very doubtful. In such a 

 study of folds and villi, difficulties present themselves which are almost 

 insurmountable without the aid of embryology. So in order to obtain 

 a correct idea of the primitive as well as the more advanced forms of 

 mucosal elevations, it is necessary not only to make rather extended 

 observations of numerous species of all great groups, but also to trace 

 the development in several types of vertebrate embryos. The possibili- 

 ties for work along these lines are very great, and in this investigation, 

 only so far as time and material would allow, such work was attempted. 

 An effort was made to determine as clearly as possible the limits of 



' This paper was presented to the faculty of Cornell University as a thesis for the 

 degree of Doctor of Philosophy, May 1902. I wish to express my grateful apprecia- 

 tion of the facilities placed at my disposal by the Department of Histology and 

 Embryology, and for the aid and encouragement of the instructing staflF. I am also 

 greatly indebted to Prof. B. G. Wilder of the Department of Vertebrate Zoology, 

 who very kindly enabled me to examine many valuable specimens which are in the 

 possession of that Department. 



