406 Dr. E. Cohn on a new genus of the family of Volvocineae. 



to examine them I had only to take out the cork, and a drop of 

 the water adhering to it furnished me with all the stages of de- 

 velopment of our organism simultaneously in very large numbers. 

 After a short time the Stephanosphcerce had again assembled on 

 the cork. I may remark in passing, that there is scarcely any 

 sight more beautiful under the microscope, than a large number 

 of these elegant crystal-like spheres crowded together, rolling 

 through the water in all directions like revolving wheels, with 

 ttieir bright green and often curiously branched wreaths of pri- 

 mordial-cells, sometimes exhibiting themselves as rings, and 

 sometimes as zones, sometimes rotating round a centre, and at 

 others rolling away in the strangest curves. 



1 made some experiments to investigate more closely the cause 

 of accumulation of the Stephanosphcera alone on the cork, and 

 this furnished me with a sufficient explanation of the behaviour 

 of the spheres towards light. When I placed water filled with 

 Btephanosphcera and Chlamydococcus pluvialis, in a shallow porce- 

 lain saucer in a window, a green band was soon found at the 

 margin of the fluid turned towards the window, and this was 

 almost exclusively composed of Stephanosphara, while at the op- 

 posite side the Chlamydococcus had collected, but with scarcely 

 a single Stephanosphmra. Since the side of the water turned 

 towards the window was kept in shadow by the projecting mar- 

 gin of the saucer, and thus constituted the darkest part, while 

 the brightest point was on the opposite side, it followed that the 

 Stephanosphara avoids the light, and accumulates at the darkest 

 part of the vessel, as is also shown by the collection under the 

 shadow of cork. When I next covered the side of the saucer 

 turned towards the window with a strip of wood, so that this 

 part was kept quite dark, while the opposite side of the saucer 

 was not overshadowed by it, within two hours all the Stepha- 

 nosph(jEi'(B removed from the darker margin at which they had 

 previously collected, yet did not repair to the opposite illumi- 

 nated margin, but arranged themselves in a green line going 

 transversely across the water, which corresponded accurately to 

 the limit between the deep- and half-shadow of the slip of wood, — 

 a position the more striking, since green microscopic plants when 

 uninterfered with collect always at the margins alone and never 

 in the middle of the water. When I placed the slip of wood so 

 that it reached across from the front to the back, from the dark- 

 est to the lightest place, the green band was seen neither on the 

 margin next the window nor on the opposite; but the green 

 cloud of Stephanosphoera soon appeared on each side of the strip 

 of wood outside its central shadow. Repeated experiments gave 

 the same result with the greatest certainty. From these facts it 

 follows, that the moving spheres of Stephanosphsera seek the darker 



