Development of intestinal coiling in anuran larvae.

In aquatic vertebrates such as the cyclostomes, fishes, and urodele Amphibia, the digestive tract develops as a relatively straight tube characteristicalaly bent toward the right side in the region of the pylorus and duodenum. Aiiuran amphibians excite particular interest because they are the most primitive vertebrate group to deve1,op a highly coiled intestine, such as amniotes generally possess. Apriori one might think of two extreme alternatives to explain the pattern of the digestive viscera. The first is that these organs are preformed within the endoderm and merely undergo histological differentiation in order to attain their typical position in the fully differentiated larva. The other alternative is that the form of the digestive organs is determined not by the endodermal lining but by the molding influence of the surrounding mesodermal derivatives. On the side of the first alternative is the abundant evidence in support of Holtfreter's conclusion that endoderm in Amphibia is regionally determined histologically sometime before gastrulation (Holtfreter, '31, '33, '38a, '38b, '39 ; Kemp, '46 ; Stableford, '48), despite claims by Kusche ( '29)' Balbinsky ( '38) and Dalcq and Halter ('43) that endoderm retains the capacity for "histological regulation'' (Holtfreter, '38a) subsequent to gastrulation. Presumptive endoderm has also been shown to possess certain intrinsic dynamic tendencies ' Aided by a grant from the Horace H. Rackham School of Graduate Studies, University of Michigan.