50 IOWA STUDIES IN NATURAL HISTORY 
crease in thickness is accomplished by rearrangement and ag- 
gregation of the elements already present. 
SUMMARY 
The period of metamorphosis and the coincident period of 
fasting occupy approximately ten days in Rana pipiens and nine 
days or less in Ambystoma tigrinum. 
Quantitative data presented indicate the following aun 
during metamorphosis: 
R. pipiens A. tigrinum 
Average reduction in total weight 57.3 per cent 28.7per cent 
Average reduction in length of stomach 
and intestine 82.2 per cent 45.8 per cent 
Average reduction in length of stomach 54.5 per cent 54.8 per cent 
Average reduction in weight of stomach 
and intestine with contents 92.8 per cent 68.6 per cent 
Average reduction in weight of liver 80 per cent none 
The more important histological changes in the stomach and 
intestine during metamorphosis consist in increasing thickness 
of the several layers and more compact aggregation of the tissue 
elements as the reduction in the length of the digestive tube takes 
place. The increase in the thickness of the several layers does not 
involve active cell proliferation, but is accomplished by rear- 
rangement and aggregation of the elements already present. 
BIBLIOGRAPHY 
Barfurth, D.—Versuche iiber die Verwandlung der Froschlarven.—Arch. 
f. mikr. Anat. 29: 1-27. 1887.—Die Riickbildung des Froschlar- 
venschwanzes und die sogenanten Sarcoplasten.—Ibid, 29: 35-60. 
Chauvin, Marie von—Ueber die Verwandlung der Mexicanischen Axolotl 
in Ambystoma.—Zeitschr. f. wiss. Zool. 27: 522-535. 1876. 
Griffith, J—Observations on the absorption of the tadpole’s tail—Jour. 
Path. & Bacteriol. 3: 131-143. 1894. 
Holmes, S. J.—The biology of the frog. 1912 
Noetzel, W.—Die Riickbildung der Gewebe im Schwanz der Froschlarve.— 
Arch. f. mikr. Anat. 45: 475-512. 
Powers, J. H.—The causes of acceleration and retardation in the meta- 
morphosis of Ambystoma tigrinum.—Am. Naturalist $7: 385-410. 
1903. 
Ratner, igi serene: des Darmes bei der Froschlarve. 1891. 
Schaper, A.—Beitrige zur Analyse des thierischen Wachsthums.—Arch. f. 
Entwickelungsmechanik I. Organismen 14: 307-400. 1902. 
