FREE INSTITUTE OF SCIENCE 



MENDEL AND HIS LAW 



another, it was possible to trace the course of any organ throughout the entire 

 length of the body. Such methods of killing, fixing, sectioning, mounting, and 

 staining have made possible the sort of investigation which Weismann has done. 

 Cells examined by methods such as this prove to have at times an exceed- 

 ingly complicated structure, particularly in that central part of the cell known 

 as the nucleus. There seems to be a general similarity between the protoplasm 

 of one cell and that of another, though this resemblance is really only general. 

 But there is evidently a very distinct individuality in the nuclei of different 

 cells, and it is in this nucleus that Weismann came to study with infinite pains 

 the processes which led him to his interesting and remarkable conclusions. 

 These changes in the nucleus occur particularly in the cells of rapidly growing 

 tissues, where cell multiplication is constantly taking place. The process 

 itself cannot be viewed under the microscope. Before a cell can be brought 

 under a power high enough to see these changes, it must be killed and stained, 

 and hence the process ends. But if a large number of cells are killed at the 

 same time, some of them are sure to be in one stage of the development and 

 others in still another. By comparing these, it has become possible to imagine 

 the process just as one looking at the film intended to produce a moving picture, 

 without power to project it upon the screen, might still see what the picture was 

 meant to convey. In a cell in its resting stage, the nucleus seems to be simply 

 more or less cloudy. As the time approaches when the cell is to divide, a 

 streaming motion in the cloudiness seems to result in the formation of a long 

 and slender thread. While this process is going on, a little spot near the nu- 

 cleus itself divides into two spots, each of which moves away from the other in 

 such way that after a while these spots are on opposite sides of the nucleus, 

 while from the spots slender rays proceed in all directions, but particularly 

 through the nucleus. On the sides which are toward each other the rays from 

 these spots meet in the nucleus, forming a spindle. By this time the slender 

 thread which was streaming around the nucleus has broken into separate 

 pieces. These seem to be laid hold of by the threads of the spindle and pulled 

 into position in the center of the nucleus, each piece of the thread being bent 

 into the shape of a horseshoe. Now comes the crucial moment. Each horse- 

 shoe splits from end to end into what look like exactly similar halves, and the 

 two halves move away from each other toward opposite sides of the nucleus. 

 Thus the material which had formed the streaming thread has been slit into 

 half from end to end and the fragments have collected in opposite ends of the 



