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The activation-desensitization sequence of muscle end-plate receptors induced by carbamylcholine has been studied in isotonic Ca2+ and isotonic Mg2+ Ringer solutions. Desensitization occurred more rapidly in the calcium solution although the extent of receptor activation was similar in both solutions. The results demonstrate that even in high concentration, Mg2+ does not exert an action on desensitization comparable to that of Ca2+.

Recently there have been various discussions of perceptual development (4, 8), response factors in learning (7), or early discrimination learning (5) which are in general concerned with an organism's early experience with stimuli and the effects of this experience on subsequent performance. Experiments with both rats (3) and chimpanzees (9) have also demonstrated that early environment is important for later discrimination learning. For practical as well as theoretical purposes it would be valuable to know the type and amount of experience that is necessary, and the time at which it should occur, if the time factor is critical at all. Young children may be particularly good Ss for such research inasmuch as discriminations can be found which they do not normally make and seem to learn only with great difficulty before certain ages. The ability to learn these discriminations may then be taken as a critical indication of the effect of various types of experience in a pretraining situation. A number of studies have been done (I, 6, io) following such a plan but, in most cases, the problempresented to the child was simply one of learning which response to attach to each stimulus, inasmuch as the basic distinction among stimuli had probably already been learned. For the present study a situation was sought where the presentation of differential stimuli apparently aroused no differential responses beyond those involved in receptor activation. This would provide a situation that would coincide more closely with Hebb's notion of early learning and would also fit closely with what others have referred to as perceptual development. A study by Swanson and Benton (ii) suggested that leftright distinctions provide a difficult learning task for young children. After

Responses of the rabbit ear artery in vitro to an agonist administered either intra- or extraluminally were enhanced by its additional application in the same concentration by the alternate route. It appears that when an agonist is applied to only one surface of the vessel it is distributed non-uniformly in the media at the time of the response plateau.

There are no methods available for the direct determination of the concentration of noradrenaline in the “biophase”, i.e., in the very small part of the extracellular space in the immediate neighbourhood of the receptors of adrenergically innervated organs. Hence, any discussion of the factors influencing the concentration of the amine at the receptors has to be based on indirect evidence. If the direct approach is impossible, one can gain some insight from recording responses (or changes in responses) of the effector organ to known concentrations of noradrenaline. However, any interpretation involves two crucial assumptions, namely that, in the absence of conclusive evidence to the contrary, the interaction between drug and receptor as well as the function of the link between receptor activation and eventual response remains unaffected by the change in experimental conditions which is being studied. In many experimental situations the validity of these assumptions remains questionable, since it has not been possible to obtain positive evidence.