488 Intestinal Folds and Villi in Vertebrates 



5. Zigzag folds (or net arranged folds). 



6. Zigzag arranged villi or otherwise regularly arranged villi; 



and closely after this: 



7. Irregularly arranged villi throughout the intestine. 



8. Villi in the small intestine and caecum, not in the large 

 intestine. 



9. Villi in the small intestine alone. 

 10. Valvule conniventes formed. 



12. By embryological study, villi are found to develop from folds in 

 the beginning, but later villi may be formed without passing through 

 the fold stage. 



13. In chicken embryos straight folds are formed first near the pylo- 

 rus, these become longer, more numerous and wavy, then zigzag, and 

 then break up into villi at the angles of the folds downwards; later villi 

 lose their zigzag arrangement and others develop without passing 

 through a fold stage. 



14. In the white rat the early development of villi is very rapid. In 

 the beginning longitudinal folds appear first in the duodenum, later in 

 other parts of the small intestine. These folds break up into villi and 

 villi are afterward formed as simple upgrowths of epithelium. 



15. A study of different species of vertebrates which have folds and 

 villi at the same time in the intestine of the adult, shows two ways in 

 which villi are formed from folds: first, from zigzag folds, the usual 

 way ; second, from net-arranged folds. 



16. Valvulge conniventes, apparently found only in man, develop 

 from simple, semicircular thickenings of the mucosa long after the villi 

 are developed. They are simple, secondary mucosal elevations and bear 

 villi on their surface. 



17. At one time during the development of the intestines of mam- 

 mals, villi are found throughout the large intestine including the caecum 

 and appendix. As the intestine grows in size and extent the villi dis- 

 appear. The disappearance of the villi seems to be for the most part 

 independent of the development of the intestinal glands or crypts of 

 Lieberkiihn. 



BIBLIOGEAPHY. 



A very complete bibliography of this subject, together with numerous 

 abstracts from many writers, will be found in Oppel's Vergleichenclen 

 niikroskopischen Anatomie, Vol. II, 1897. 



1868. Bakth. Beitrag zur Entwickekmg der Darmwand. Sitzungsber. 

 d. Wien. Akad. d. Wissensch., math. nat. KL, Bd. LVIII, 2 Abt., pp. 128-136. 



