THE MECHANISM OF LIFE 527 



with the substance in question ; in the latter case a positive thigmo- 

 taxis exists, since the cell endeavours to extend as much as possible 

 the surface of contact with the food -body and surrounds the latter 

 with its protoplasm. These two factors are very frequently united. 

 But the ball of food is always surrounded upon all sides by 

 the protoplasm, if it has come into contact with the latter, whether 

 upon the surface of a naked protoplasmic body, or in the mouth - 

 opening of an infusorian cell. The process of surrounding it is 

 explained very simply from the expansory effect which the stimulus 

 of the food-ball exercises upon the protoplasm ; if the surface of 

 the latter rises up- around it, it must finally be surrounded by the 

 protoplasm. The ingestion of solid food, therefore, finds- it's ex- 

 planation in the mechanism of chemotactic and thigmotactic re- 

 actions; we have already become acquainted with these in detail else- 

 where. 1 How solid substances are given off is still little understood. 

 This appears to be left more or less to chance. At least this is 

 the impression obtained from Amoeba. As a rule the solid sub- 

 stances lie in vacuoles, and, if by the continual movement of the 

 protoplasm the vacuole is brought close to the surface, the thin 

 wall that separates it from the surrounding medium occasionally 

 breaks, and the contents are set free. But perhaps stimuli of 

 .some sort coming from the excreted particle are necessary for this 

 rupture of the wall. The questions whether the removal of the 

 excretion through the anal opening, which occurs in infusorian cells, 

 likewise depends upon stimulation, must be left until the process 

 has been studied more in detail. 



A remarkable phenomenon, which has often been cited as afford- 

 ing special difficulties to a mechanical explanation, is the so-called 

 selection of food on the part of certain cells, i.e., the fact that certain 

 cells take up only certain substances among all those available. 2 

 Thus, regarding the seeking of ^ro^yra-threads by Vampyrella 

 Spirogyrae and the selection of fat-droplets from the intestinal con- 

 tents by the intestinal epithelium-cells, Bunge ('94) says : " No 

 chemical explanation of these phenomena is conceivable." But 

 why this should be so is not easily understood. If in these ap- 

 parently isolated phenomena the fact upon which they are based is 

 clearly understood, i.e., that every cell takes up certain substances 

 and not others, the action of the cells is self-evident. Every cell 

 has its characteristic composition and its own peculiar metabolism. 

 Is it not then comprehensible that only those substances are drawn 

 from the medium into the metabolic circulation of the cell that 

 have chemical relations with the constituents of the cell-body and 

 are necessary to the maintenance of metabolism, while others which 

 have no such relations with the living substance and are indifferent 

 to the cell, are not taken up and, when free locomotion is possible, 

 are not sought out ? The principle upon which this phenomenon 

 1 Cf. p. 498. 2 Cf. p. 146. 



