40 III. PROTOPLASMIC MOVEMENTS MICRO. DRAWINGS. 



Most of the threads end in the plasma layer surrounding the 

 nucleus. The current in any single thread moves often only 

 in one direction ; often, however, it can be seen that even in very 

 thin threads there are two currents in opposite directions. The 

 movement is recognisable by , the microsomes and leucoplasts 

 borne in the clear hyaloplasm. With continued observation it 

 will be seen that the strands slowly change their thickness and 

 arrangement. New connecting threads form, older ones become 

 gradually thinner in the middle, finally break through, and with- 

 draw into other strands. Thus by degrees the conformation 

 changes. The nucleus is almost spherical, or often oval or some- 

 what flattened. With the strongest magnification which is at 

 our command it appears finely punctate, and in it can be readily 

 distinguished some larger granules (Nucleoli). Occasionally two 

 nuclei lie close together in such a cell, because the original 



FIG. 15. Abbe's Drawing Camera, No. 44a (Zeiss) ; two-thirds natural size. 



nucleus has divided. The nucleus is towed about hither and 

 thither by the cytoplasmic strands, and thus slowly changes its 

 position in the cell. In order to prove this, take rapidly a sketch 

 of the cell, and compare this with the arrangement of the nucleus 

 and the threads after the lapse of some time. Accurate sketches 

 can only be taken by means of a drawing prism, and only such 

 have real value for later comparison. We will, therefore, endea- 

 vour now to become acquainted with the use of the drawing prism. 

 Drawing Appliances. Of all drawing appliances for use in 

 daylight, and not necessitating the removal of the body of the 

 microscope from its vertical position, the most perfect is Abbe's 

 Camera Lucida, manufactured by Zeiss of Jena. It is made in 

 several forms, of which one is illustrated in the adjoining Fig. 15. 

 The apparatus is placed upon the tube of the microscope, 



