460 Intestinal Folds and Villi in Vertebrates 



folds and villi; to describe the character of folds and villi in some 

 forms not already described as well as to supplement what was known 

 about others; to determine, as far as possible, the form of folds and 

 villi characteristic of different groups of vertebrates; to throw some 

 light upon the development of villi in different forms — first, by the 

 general survey of conditions existing in adult species; second, by em- 

 bryological study of a few types. 



Methods. 



In order to obtain folds and villi in their usual form, fixation must 

 be used which will not cause the muscular fibers of the fnuscularis 

 mucosce to contract and so produce unusual shapes. The intestines 

 were usually injected and placed in the hardening fluids before the 

 tissues had ceased to be irritable, so almost any of the fixing fluids 

 which are rapid in their action were not desirable. Although formalin 

 acts quickly, it was found to be very satisfactory, as the villi appear 

 only slightly distorted, but it was not so useful when the finer structural 

 elements were to be preserved. Miiller's fiuid, or a solution of 3 per 

 cent dichromate of potash, does very well for preserving villi, in that 

 the action of these fluids is slow and villi are seldom contracted; but 

 when these fluids are employed, the epithelium is usually exfoliated. 

 At times a rather high percentage of alcohol was efficient as a preserva- 

 tive, as the action was not rapid. 



After hardening the intestines either in the fixing fluid or in alcohol, 

 rather thick sections were cut in collodion; the thickness of the sec- 

 tions depending upon the size of the villi to be examined. Examina- 

 tions of the whole intestine under liquids often helped to determine 

 the character of folds or villi. When the muscular coats were thin 

 enough, bits of intestine were stained with hydrochloric acid carmine, 

 and mounted mucosa up. When this was not possible, villi were exam- 

 ined which had been isolated by scraping or cutting from the mucosa. 



For counting and measuring folds and villi, various methods were 

 used. The most satisfactory, but not always applicable, method was 

 by means of an ocular micrometer in squares. When villi were very 

 numerous and the walls of the intestine thick, they were counted by 

 removing a measured bit of mucous membrane and isolating the villi 

 from it, counting their tips or bases until by repeated trials, uniform 

 results were obtained. In every case the intestines were studied by 

 as many different methods as possible and the results compared. 



The following is a list of the methods ordinarily used for each 

 specimen : 



