6 INTRODUCTION TO CYTOLOGY 



Ringer's solution or paraffin oil, the streaming of the protoplasm often 

 continuing actively for hours or days. In the remarkable researches of 

 R. G. Harrison and others^ it has been shown that cells from various 

 tissues of vertebrate animals can be isolated and kept actively growing 

 and multiplying in culture dishes for a length of time often far exceeding 

 the normal life period of such tissues in the body. Under such conditions 

 the cells appear pale and colorless, but by careful adjustment of the 

 light the other components may be made to stand out with considerable 

 clearness. The cytoplasm usually appears as an optically homogeneous 

 fluid carrying various sorts of small inclusions which may show Brownian 

 movement if the viscosity of the cytoplasm is not too high. Chondrio- 

 somes appear as dull granules or threads somewhat difficult to make out 

 in many cells, while oil droplets stand out as highly refractive bodies. 

 Centrosomes and centrioles may often be seen, especially if identified by 

 surrounding astral rays. The nucleus may appear optically homogeneous 

 except for the nucleolus, which is usually seen without difficulty, or it 

 may present a finely mottled ("granular") appearance. In favorable 

 material the reticulate structure is evident. The cell sap of plant vac- 

 uoles appears as a clear space, with or without inclusions. When properly 

 illuminated in a dark field, many living cells are strikingly beautiful 

 objects, the chondriosomes and other small bodies appearing as glistening 

 specks in the pale protoplasm. 



The visibility of certain components of living cells may be enhanced by 

 the use of dyes which enter the cell without killing it. Examples of such 

 "vital dyes" are neutral red and Janus green, which stain vacuoles and 

 chondriosomes respectively. The vital staining of the nucleus with 

 several dyes is also reported,^ but in several such cases it has been ques- 

 tioned whether or not the stained nuclei were actually uninjured and 

 wholly normal. 



Particularly noteworthy are the results obtained by a number of 

 workers^ through the use of micromanipulative methods. Apparatus 

 and technique have been developed whereby it is possible to dissect, 

 inject, and otherwise operate upon living cells under very high powers of 

 the microscope. Many cells are highly tolerant of such treatment. A 

 microneedle may be pushed carefully into the cell and even into the 

 nucleus of a Tradescantia stamen hair without causing visible injury or 

 cessation of activity, but a lowering of the internal pressure due to escape 

 of material from the cell when the needle is withdrawn causes a series 

 of alterations from which the cell may later recover (Martens and Cham- 

 bers). In such ways it has been possible to learn much about the natural 



' Carrel, Leo Loeb, Burrows, H. V. Wilson, W. H. and M. R. Lewis, Drew. 

 4 Kiister (19186, 1926), Gicklhorn (1927), Paltauf (1928), P. Dangeard (1923o). 

 « Kite (1913), Chambers (1914 et seq.), Seifriz (1918 et seq.), C. V. Taylor (1920 

 et seq.), Peterfi (1923), Martens and Chambers (1932). 



