Comparative studies were made of the exocrine pancreas in normal, thyroidectomized, and thyroxine treated Rana pipiens tadpoles during various stages of development. The weight of the pancreas as milligram per cent body weight (mg% B.W.) is greatly reduced during both artificially induced and natural metamorphosis. During natural metamorphosis the reduction in mg% B.W. of the pancreas begins in the early larval stages and continues in a consistently decreasing pattern through the final stages with the maximum reduction (69%) occurring by the mid-larval stages (XVI-XX).

In thyroxine treated tadpoles, reduction in mg% B.W. of the pancreas is directly proportional to hormone concentration and the relative weight of the gland is significantly less than that observed in any of the controls of equivalent limb stages even though the absolute weight of the gland has increased over that of the earliest stages (I–V). The pancreas of the thyroidectomized tadpoles showed no significant difference in mg% B. W. when compared to that of a normal control of equivalent limb stage range. Histological investigation of the pancreas revealed no evidence of widespread cellular necrosis of the exocrine elements. Some acinus disorganization, typified by large spaces in the connective tissues and a syncytial nature of the acinar cell cytoplasm, can be seen in the later stages. A discussion of the possible role of thyroid hormone in the normal development of the pancreas is presented. It is concluded that reduction in the mg% B.W. of the pancreas is a naturally occurring phenomenon during metamorphosis resulting from intestinal evacuation and water loss. The water loss may be influenced and regulated indirectly by thyroxine through an effect on cellular and subcellular membrane permeability.