424 PHYSIOLOGY OF GROWTH. 



obtain, being found in stagnant water, street gutters, etc. In 

 order to obtain very vigorous individuals for examination, I first 

 cultivate the Euglena material for a few days on bits of turf 

 placed in a dish containing food solution such as used in water- 

 culture experiments. The fragments of turf are only about half 

 immersed in the fluid, and the green Euglena material is simply 

 placed on their moist upper surface. The culture vessels are 

 left for a few days before a window with a south aspect, and the 

 fragments of turf are then placed in a porcelain dish, flooded with 

 spring water, and left in the water for a few hours. During this 

 time many Euglena swarmers collect in the water. Examination 

 under the microscope shows that the body of Euglena viridis is 

 spindle-shaped. Nucleus and chlorophyll bodies are present. 

 At the anterior end, which bears a long cilium, are seen vacuoles 

 and a red eye spot. According to Klebs the Euglena is clothed 

 throughout its life by a membrane, and under favourable external 

 conditions the Euglenae pass into a resting state, in which they 

 are immotile. The free forward movements of the EuglenaB are 

 effected by means of the cilium, and are always accompanied by 

 a rotation of the whole organism. In order to follow exactly 

 these movements of progression, we dip a tube into the water 

 containing the bits of turf, and introduce some of the swarmers 

 into the hanging water drop of a small moist chamber, and 

 observe under the microscope (as regards the preparation of the 

 moist chamber, see 138). The swarmers are capable of move- 

 ment both in complete darkness and in the light. The light 

 nevertheless exerts a directive influence on the movement of 

 Euglena, which therefore comes under the category of photo- 

 tactic organisms. At the commenceTiient of our experiment the 

 swarmers are distributed pretty uniformly in the hanging drop of 

 the moist chamber, but it can easily be determined under the 

 microscope, that nearly all of them, especially if we cut off the 

 light reflected from the mirror, very rapidly collect on the side of 

 the drop towards the window, i.e. towards the source of light. If 

 we rotate the slide, with the moist, chamber, through an angle of 

 180, the swarmers again fall into lively movement, and endeavour 

 to reach once more the margin of the drop which is directed 

 towards the source of light. We can, however, only observe these 

 phenomena when the light to which the swarmers are exposed 

 is not too intense. With more intense light, most of the swarmers 

 collect, not at the illuminated edge of the drop, but at the 



