PHYSIOLOGICAL 371 



visible; it is not seen, it merely betrays its presence by scattering 

 light rays. 



Another new method which has notably increased our know- 

 ledge of protoplasm is the "micro-dissection", which is especially 

 associated with the names of Kite and Chambers. Some of the 

 early naturahsts, before microscopy began to be an everyday 

 method, were masters in the art of minute dissection, unravelling 

 the food-canal of a flea or separating out the intact brain of an ant, 

 which Darwin called the most marvellous atom of matter in the 

 world. With good eyes, steady hands, sharp needles, and no end of 

 patience, it is possible to do very extraordinary things, but there 

 are obvious limits to direct manipulation. It is on indirect manipula- 

 tion that the new method of micro-dissection depends, and many of 

 its successes are very striking. Prof. Robert Chambers of Cornell 

 University is able to inject an Amoeba, which is just visible to the 

 naked eye as a white speck against the dark mud. It is often about 

 a hundredth of an inch in diameter. From the centre of an egg-cell 

 even smaller than this, Prof. Chambers is able to jerk out the 

 nucleus, and he can divide the same cell into several pieces. This 

 has been done before, but it may be noted that in most of the early 

 experiments with separated-off pieces of egg-cell, those of Prof. 

 Delage, for instance, the method used was not micro-dissection but 

 shaking the egg-cells in the fluid in which they were floating. 



How does Chambers effect his manipulative marvels? The object 

 to be micro-dissected is included in a "hanging drop" suspended 

 from the under-side of a cover-glass firmly fastened on the stage of 

 the microscope in the field of vision. The cover-glass forms, as it 

 were, the roof of a moist chamber that has no floor. Into this space 

 there protrude the exquisitely fine tips, straight or cmrved or bent 

 at right angles, of glass needles which are fixed in holders. These 

 holders again are attached to an ingenious system of screws fastened 

 on the under-side of the stage of the microscope. The object cannot 

 be seen with the naked eye, but it is quite clear in the centre of the 

 field of the low-power or high-power microscope, and so are the 

 points of the needles. By working the screws it is possible to move 

 the needles up or down, as well as in a horizontal plane, and it is 

 not difficult to bring the very delicate tips together. Thus one tip 

 may be used to press the cell against the firmly fixed cover-glass, 

 while the other is used to cut a piece off. It is possible in this way 

 to isolate a particular portion of a cell and to watch what happens. 

 The method can also be used to good practical purpose in isolating 

 a single micro-organism, so that an absolutely pure culture or "pure 

 line" can be secured. It is probable that this method of micro- 

 dissection has a great future before it. 



Our present question, however, is : What does the method indicate 

 in regard to the nature of protoplasm? AU the evidence points to 



