5 6 BASIC METHODS FOR EXPERIMENTS 



Similarly, the fixative employed for the cytoplasm, the 

 menstruum in which the cytoplasmic inclusions lie, should be 

 one which gives a picture that simulates the cytoplasmic struc- 

 ture of the living cell. Such a fixative should be as perfect for 

 the cytoplasm holding the inclusions as for the cytoplasm that 

 has been freed of inclusions. Depending upon the object 

 studied, the fixative may or may not be the same as that which 

 fixes the nucleus or the inclusions of the given cell. 



At the surface of an o:^^ the cytoplasm has structure which 

 differs from that more centrally located. This, the ectoplasm, 

 so highly mobile, registers changes in the environment and 

 reveals differences in structure that run with the activity of the 

 cell. Its structure as seen in the living egg is faithfully repro- 

 duced only with the aid of certain fixatives. 



There is no one fixing solution that will serve for all struc- 

 tures found in an egg. This statement is true for animal cells 

 in general. Moreover, that a named fixing solution gives excel- 

 lent results when used on one species of &ggy does not mean that 

 it will succeed on another. My first suggestion, therefore, is 

 that the zvorker should try out several fixing solutions in order to 

 ascertain that which is best for his purpose, always of course 

 comparing the fixed egg with the living. 



A very simple observation that I have made may be men- 

 tioned. Let us take a few developing eggs mounted in a hollow 

 microscope slide under low power of the microscope and run 

 on to them a fixing solution, noting the time in seconds when 

 the eggs are killed without any change in form. If, for example, 

 we take drops of an egg suspension at the time when the first 

 cleavage furrow is appearing and try out fifteen to twenty fixing 

 solutions, we note that in some cases the blastomeres change in 

 shape before they are killed; in others, the blastomeres are 

 killed in exactly that stage in which they were when subjected 

 to the solution. For the echinid egg I have found that some 

 descriptions of cell division based on fixed eggs are at fault 

 because of the use of a fixing solution which alters the shape of 

 the egg. 



Although, in general, the volume of the fixing solution plays 

 a part in determining results obtained, this is not the case in 

 the foregoing observation, because the same result is obtained 



