THE VITAL PHENOMENA OF THE CELL 239 



way in which I explain the interesting observations of Klebahn 

 (VI. 43) upon two species of low Algae, Closterium and Cosmarium. 

 A more detailed account is given in the chapter on the process of 

 fertilisation, p. 279. 



IV. Influence of the Environment upon Cell-Division. 



The complex play of forces, exhibited to the spectator at each cell- 

 division, can, just like the phenomena of protoplasmic movements, 

 which have been already described, be influenced to a considerable 

 extent by external agencies. Only here, for obvious reasons, the 

 conditions are more complicated than with the protoplasmic move- 

 ments, because bodies differing in structure, such as protoplasm, 

 nuclear segments, spindle threads, centrosomes, etc., are concerned, 

 and these can be altered in very various ways. As yet very little 

 experimental work has been done upon the subject. If the ques- 

 tion is raised as to how the processes of nuclear division are 

 affected at any individual stage by thermal, mechanical, electrical 

 or chemical stimuli, the answer is but unsatisfactory. The most 

 complete experiments that have been made at present have been 

 upon Echinoderm eggs, whose reactions during division to thermal 

 and chemical stimuli have been carefully observed. 



It is generally accepted that the rate of cell- division is affected 

 by the temperature, but what are the exact maximum and 

 minimum temperatures, and what changes in the nuclear figure are 

 produced by temperatures exceeding the maximum, have not yet 

 been accurately determined. 



I (VI. 32, 33) have conducted a series of experiments upon the 

 influence of temperature from 1 to 4 Celsius below zero. 



If dividing Echinoderm eggs are cooled down for about 15 to 20 

 minutes from 1 to 4 Celsius below zero, after a few minutes 

 the whole achromatin portion of the nuclear figure becomes dis- 

 integrated and destroyed, whilst the chromatin portion forming the 

 nuclear segments experiences only small or unimportant changes. 

 The most instructive figures are seen when the nuclear segments 

 are arranged in the equator (Fig. 128 A), or when they have 

 already migrated to the two poles, as can be seen from Fig. 128 

 B-, the protoplasmic radiations arid the spindle threads have abso- 

 lutely disappeared, whilst the radiation spheres in the neighbour- 

 hood of the centrosomes are marked by bright portions in the 

 yolk. The nuclear segments alone are unaltered in appearance 

 and 



