EXPERIMENTS ON CLEAVAGE '2(53 



the nucleus which also may be effective if separated from the 

 nucleus. 



8. The physiological causes of the segmentation of the 

 nucleus are not directly touched by these experiments. But 

 two points ought to be mentioned : first, that the segmenta- 

 tion of the nucleus in the concentrated solution (2 g. of NaCl 

 to 100 c.c. of sea-water) was retarded, and at last ceased 

 entirely after from twelve to twenty-four hours; secondly, 

 that the segmentation of the nucleus was extremely irregular 

 when the protoplasm did not take part in segmentation. We 

 see in these facts some of the influences which the proto- 

 plasm exercises on the segmentation of the nucleus. This 

 influence may be exercised in this way, that by the high 

 intra-cellular pressure which normally exists in the cleavage 

 spheres these spheres press and flatten each other. The form 

 of the cell, however, determines, as Sachs showed long ago, 

 the orientation of the plane of division, and, as Hertwig 

 believes, in such a way that the longitudinal axis of the 

 Kernsplndel is put in the longest diameter of the cell. 

 Therefore we ought to expect that, within certain limits, with 

 increasing intracellular pressure, Sach's law of the rectangu- 

 lar division of the planes of cleavage would become^ more 

 obvious. I found, indeed, that in normal, or still more in 

 somewhat diluted, sea-water, where the turgor, and conse- 

 quently the flattening of the cleavage spheres, was the great- 

 est, Sach's law was the most exactly realized. Therefore this 

 geometrical regularity in the segmentation of the nucleus 

 which is so striking under normal conditions must disappear 

 at once if the protoplasm does not take part in segmentation. 



9. Our observations concerning the dependence of irrita- 

 bility of the protoplasm upon the water contained in the tis- 

 sues add one more fact to those given already to explain the 

 importance of water for all processes of growth and develop- 

 ment. If we reduce the amount of water in a regenerating or 



