ONE FIQUBE
In a series of studies Wulzen ('23, '26, '27, '29) has shown that certain tissues ; viz., raw liver, kidney, thymus, adrenal, and brain, when fed to Planaria maculata, led to excellent growth. On the other hand, raw beef muscle and egg yolk had but little, if any, effect. Raw egg white or a mixture of this and starch were likewise ineffective. Egg yolk permitted slight growth, while a combination of egg white and yolk was superior to yolk alone. The amount of growth-promoting factor present in the several tissues was decreased by heating.
On adding an ether extract of liver, it was found possible to restore to brain a part of the growth-promoting activity which was lost on heating. In collaboration with Bahrs ('28, '29, '30, '31), it was shown further that pancreas has no effect in promoting growth. Of the various regions of the kidney the cortex was best, the boundary zone of the medulla next, and the papillary zone least in promoting growth. Various portions of the digestive mucosa of rabbits were likewise found to vary in their growth-promoting properties. Decrease in the activity of tissues resulted when they were permitted to remain at low temperatures for some time.
The advantages planarians offer as animals for nutritional studies are: Economy in food consumption, the ease with which they may be kept in the laboratory, the large number of experiments which may be carried out, and the rapidity of 375