The aim of this study is to investigate the strategies employed by advanced high school and university students working on six tasks concerning comparison and conservation of area. Special care has been taken in the test design so that the problems could be dealt with using a variety of solution methods. Written responses and in-depth interviews with 21 12th graders and university students of mathematics provided the empirical data. The results show that the majority of the participants either did not prefer or had difficulties employing adequately formal reasoning. Visualization factors seem to exert considerable influence. Moreover, many students confuse congruence with area equivalence.

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IS WELL KNOWN that cross-clamping of the abdominal aorta, both experimentally and clinically, results in a drop of the systemic pressure, following sudden release of the occluding clamp. Various experimental and clinical studies have been done in order to explain the phenomenon. Although there is a controversy between the various authors concerning the mechanism and the various affecting parameters, there is a rather unanimous conclusion, that the declamping phenomenon is due to temporary shifting and pooling of a certain amount of blood volume towards the distal part of the body, as a result of a sudden decrease of the peripheral vascular resistance.It has been reported that the declamping phenomenon can be minimized or eliminated by using sympathomimetic amines distally to the occluding clamp.1'3'4 Pneumatic envelope to the distal part of the body has also been successfully used to prevent the phenomenon.2 It is also well known that a gradual release of the occluding clamp can minimize or prevent the phenomenon. These attempts to explain exact mechanism of the phenomenon are inconclusive and are strictly related with the distal part of the vascular bed.An attempt has been made here to interpret the phenomenon by studying it in two different perspectives. First, with simultaneous occlusion of the inferior vena cava, in order to determine blood volume relationships, and second, the declamping phenomenon in relation to the baroreceptor reflexes.Material . Two parameters were monitored throughout the experiments; systemic aortic pressure via a plastic catheter inserted through the right carotid artery into the aortic arch, and central venous pressure, via a second plastic catheter inserted through the external jugular vein into the right atrium. Both catheters were connected to pressure transducers, and the pressures were recorded. The abdomen was opened though a midline incision and the abdominal aorta in all dogs was exposed. In the dogs in Group I, the abdominal aorta was completely dissected free between the renal arteries and the aortic bifurcation, and the inferior vena cava was also dissected free between the renal veins and common iliac veins in dogs in Group I. Meticulous hemostasis was maintained throughout the dissection. After the dissection was completed, Sodium Heparin, (3 mg./Kg.) was given intravenously. The occluding clamps of the aorta and inferior vena cava were applied just distally to the renal arteries and veins.In the dogs in Group II, the aorta was cross-clamped for 10 min each time and the declamping phenomenon was produced in successive steps following the procedures listed below:1. No dissection of the aorta.805