fications of these intra-cellular movements and to abnormal types of cleavage and development—in short, these movements show that the fertilized egg is differentially sensitive.
In the further course of development particular portions of the embryo become especially sensitive to some kinds of stimuli, while other portions become sensitive to others. In this way the different sense organs, each especially sensitive to one particular kind of stimulus, arise from the generalized sensitivity of the oosperm, and thus general sensitivity, which is a property of all protoplasm, becomes differential sensitivity and special senses in the process of embryonic differentiation. Such sensitivity is the basis of all psychic processes: sensations are the elements of the mind.
3. Tropisms, Reflexes, Instincts.—All the responses of germ cells, and of the simplest organisms, to stimuli are in the nature of tropisms or reflexes, that is, relatively simple, automatic responses. Such tropisms or reflexes are seen in the movements of bacteria, protozoa and
Fig. 19. Reactions of Paramecium to Heat and Cold. At a the infusoria are uniformly distributed in a trough, both ends of which have a temperature of 19°; at b the infusoria are shown collected at the cooler end of the trough; at c they have collected at the warmer end of the trough. (From Jennings, after Mendelssohn.)
many higher animals and plants as well as in movements of spermatozoa, the movements of the protoplasm in egg cells and embryonic cells, the movements of cells and cell masses in the formation of the gastrula, alimentary canal, nervous system and other organs. Indeed the entire process of development, whether accompanied by visible movements or not, may be regarded as a series of automatic responses to stimuli.
