360 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and precisely measured. The responses to the stimulation produced by 

 two beams of different intensity acting simultaneously on opposite sides 

 of the same animal were measured in angular deflections from an initial 

 path of locomotion. 



When the opposing lights were of equal intensity, the average trail 

 of the "standardized" larvre (those showing a uniform degree of 

 sensitiveness) was within 0'09° of the perpendicular to the line 

 connecting the sources of light. When the opposing lights were un- 

 equal, the "average trail" showed a deflection toward the weaker light. 

 The amount of the deflection was definite and constant, within the 

 limits of experimental error, for a given intensity difference between 

 the lights. 



The phototactic response of the blowfly larva depends, to a large 

 extent, on the stimulating effect of constant light intensity. The re- 

 action to light of constant intensity follows the Bunsen-Roscoe Law. 

 The evolution of phototaxis is the result of the development, in con- 

 nexion with photokinesis, of certain factors which modify the action of 

 light on the organism, or indirectly distribute its effects. The critical 

 factors of phototaxis are : — 1. A distribution of the stimulant in the 

 field such that a change in axial position on the part of the animal in- 

 volves a change in the distribution, or intensity, of the stimulant acting 

 on the animal or on its sensitive surfaces. 2. The presence, within the 

 organism, of a mechanism adapted to the reception of differential 

 stimulation, and a transmitting and motor apparatus that produces definite 

 locomotor movements proportional to the intensity of the stimulation. 



The "response factor" maybe present in the form of a bilateral 

 mechanism, or in the form of a unilateral mechanism that reacts to 

 both sides of the environment because of a rotational method of loco- 

 motion. If we include under phototaxis any reaction which involves a 

 definite axial orientation with reference to light, the tropism may be 

 regarded as a special form of phototaxis, in which the response depends 

 on the bilateral structure of the mechanism of response. 



Centrifuging Spermatocyte Cells of Notonecta.* — Ethel Nicholson 

 Browne has centrifuged the testes of Notonecta in small glass tubes 

 with a little Ringer's solution. Thereafter the testes were teased out 

 on a slide or sectioned. The normal resting spermatocyte cell has 

 scattered mitochondria of two sorts, fibres and spheres, and the nucleus 

 contains a deeply staining karyosphere in which the chromatin is 

 jllected. When a cell of this sort is centrifuged, the mitochondria are 

 driven to the distal pole (away from the axis of the centrifuge), where 

 they appear as a deeply-staining mass in which individual bodies cannot 

 be distinguished except along the edge. The mitochondria are there- 

 fore the heaviest material in the cell. The rest of the cell is filled with 

 clear cytoplasm. The nucleus goes to the centripetal pole (nearest the 

 axis of the centrifuge), showing that it is of less specific gravity than 

 the cytoplasm. The karyosphere is always driven to the distal end of 

 the nucleus, and in some cases, in fact, it is driven through the nuclear 



* Journ. Exper. Zool., xvii. (1914) pp. 837-40 (1 pi.). 



