Histochemical application in a living animal was tested on the paraboloid of the accessory cone of the chick retina. After the anterior part of the eye had been cut with a Graefe's knife under ether anesthesia, the posterior part was filled with the medium for phosphorylase under exposure to light. The specimens were embedded for routine electron microscopy and the paraboloid of the accessory cone was observed by electron microscopy. Polyglucose particles were synthesized from glucose-l-phosphate in the paraboloid by the activities of phosphorylase and branching glycosyl transferase and found to be in the cytoplasmic matrices. These particles were larger in size and better stainable with lead citrate than those found in the paraboloid of the retina incubated in the medium in vitro by the conventional histochemical method. Overproduction of polyglucose particles was not found in the paraboloid of the retina incubated in the medium in vivo. These findings suggest that polyglucose particles synthesized in vivo have a close resemblance to native glycogen particles and that glycogen metabolism is regulated by the living cell. Glycolysis may not be related to the membranous structures. Therefore, application of enzyme histochemical techniques to the living organ can demonstrate more accurate morphological aspects of metabolism in the cell.