Physical Properties of Protoplasm . 149 



required for their dissection is a possible source of error that may 

 appear, at first sight, to be very difficult to control. Certainly 

 many biologists hold the view that cells rapidly undergo important 

 morphological changes following mechanical injury. With few 

 exceptions it has not been found difficult to follow the structural 

 changes that occur in cells that are being dissected; but the really 

 remarkable fact is the marked slowness of such death changes 

 as granulation, fragmentation and general coagulation, following 

 mechanical injury. 



It seems best to limit this introductory paper to a description of 

 selected types of widely different cells and in future publications to 

 treat systematically, selected types of the principal phyla of animals 

 and the chief groups of plants. 



The special literature bearing -on this investigation will be dis- 

 cussed in subsequent papers. , 



A review of such well-known theories as those of Butschli, 

 Flemming and Altmann lies outside the province of this paper. 



METHODS AND MATERIAL 



The development of a really adequate method for the dissection 

 of living cells, under the highest powers of the microscope, has made 

 possible this study. The principles of this method are simple. The 

 dissecting instrument is a glass needle that may measure less than 

 one micron in diameter and is drawn on the end of a piece of special 

 Jena glass tubing about 200 mm. long and 4 mm. in diameter. The 

 needle is held in a three-movement Barber pipette holder. The cell 

 chosen for dissection is mounted in a hanging drop in an open-end 

 moist chamber and held in place by water-glass surface tension, 

 which can be varied at will. 



Both diffuse sunlight and artificial light are used as sources of 

 illumination. For the latter a Nernst Glower has been found satis- 

 factory, but all the light waves outside of 450 and 670 p>p* are cut out 

 by the use of appropriate ray screens. The same means is used to 

 remove enough of the orange and red rays to make the transmitted 

 light perfectly white. Such light is composed of the waves that are 

 least injurious to living cells. A special condenser 15 of a focal dis- 

 15 The condenser was made by E. Leitz & Co., Wetzlar. 



