THE POSITION OF CYTOLOGY IN BIOLOGICAL SCIENCE 7 



of this character, e.g., those on cell division, have been made with the 

 motion-picture camera. 



Another tool now in common use among cytologists is the X-ray tube. 

 The irradiation of living material is now one of the best experimental 

 means of inducinji- alterations in chromosonK^s for the ]-)urposo of anal3'zing 



Fig. 3. 



^ \:m^mmit^ if 



-Cells (fibrobla.sts from heart of chick embryo) descended from a tissue culture 

 begun bj' Carrel in 1912. (Photograph by A. Carrel; after W. Seifriz.) 



the role of these bodies in development and heredity. X rays have also 

 served to reveal the ultramicroscopic structure of plant cell walls. 

 Radiations of various other types are being employed with success in 

 connection \\\ih. a number of such fundamental problems. 



Fig. 4. — Photographs of cells being operated upon with a micromanipulator. A, B, C, 

 stages in the stretching of a red blood corpuscle from an amphibian between two micro- 

 needles. D, a micro-needle entering a living plant cell. {After W. Seifriz.) 



The chief advance in microscopy so far in this century is the invention 

 of the electron microscope. Instead of making use of visible light focused 

 by a series of glass lenses as in the ordinary microscope (photomicro- 

 scope), this new instrument employs electron streams brought to a focus 

 by a series of magnetic fields. The result is a greatly increased resolving 



