Studies concernccl with the numbcr and size of renal $10meruli have been retarded for want of accurate rapid methods. Below are described results obtained by means of snch methods designed a ) for measuring the size of fresh glomeruli suspended in serum, and b) for their enumeratioii in uninjected kidneys : the latter method is dependent for glonierular identification upon the foimation of hematin in tlw capillary loops.The results show sufficient regularity to permit the coilstruction of empirical formulae for the relations of glomeriilar size and number, kidney weight, and body weight over a range of mammalian species extending: from the mouse to the elephant. MET HODSGZomfJmZar nieaszivcwezit. It is well known that Romm~ii's capsule is acicl-i.esistant ; the pi*esciit method is based up011 the fact that is also resists trauma. Bits of fresh kitlney are ground briefly by means of a mortar and pestle in 1 to 3 cc. of serum (preferably from the same animal). The amount used varies from one to six whole kidneys from newborn rats 'This work was aided by a grant from the Rockefeller Foundation. 507

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In anesthetized dogs, electrograms were recorded simultaneously from six atrial epicardial sites and a limb or esophageal lead in order to study the mechanism of atrial flutter induced electrically after an intercaval crush in comparison with atrial tachysystole induced by aeonitine. In the former, with limb lead records resembling classical human flutter in form and regularity, activation occupied most of the atrial cycle and progressed in sequence caudally in the right atrium and cranially in the left (counterclockwise flutter); flutter was usually terminated by another crush extending from the initial one to the right atrioventricular junction. The most prominent wave in limb and esophageal leads coincided with left atrial activity. Sometimes activation proceeded clockwise. The two atrial appendages may be activated simultaneously during counterclockwise flutter. These findings support the circus movement hypothesis as the mechanism of pure atrial flutter. In contrast, with aeonitine tachysystole, activation proceeded over both atria at once and was so brief that it left most of the slightly irregular atrial cycle electrically silent. Although limb leads after aeonitine sometimes resembled those of flutter, the contrast in direction and duration of excitation demonstrates profound physiological differences between the two arrhythmias.