ON EGGS OF MARINE ANIMALS 55 



The investigator by such study can correlate the changes 

 observed in the living system more exactly by comparing the 

 changed states in the fixed. The rapidity with which many 

 events in a developing egg take place is such that one can not 

 easily follow them with assurance in the living. If, however, 

 the worker has repeated his observations on the living, he can 

 by use of fixed stages come to surer knowledge as to the sequence 

 of events and to the predominating factor both of which may 

 escape him in direct observation on the living egg. Indeed, in 

 some cases, as for example, changes in large or in opaque eggs, 

 he can follow these changes exactly only in fixed preparations. 

 A great deal of work in experimental embryology and even 

 that in physiology could be much improved by correlating the 

 changes examined in the living state with those revealed by the 

 fixed. 



The Choice of the Fixative 



The choice of the fixative employed is of paramount impor- 

 tance. Since the fixed egg is a dead egg, the method of bring- 

 ing about the death-change is a primary consideration: the aim 

 is to secure fixation which will distort the structural organiza- 

 tion of the egg in the least possible degree. Thus, the fixative 

 should act very rapidly; it should penetrate quickly and yet 

 without violence; it should fix evenly, without disrupting the 

 relation of the various regions in the cell. This applies to any 

 fixing agent used, regardless of the cell component studied. 



A second consideration concerning fixation involves the 

 cellular component under study. If we are interested in nucleus 

 and chromosomes, the fixative employed is that which most 

 faithfully preserves their structure. Although these are per- 

 haps the cell structures that can be fixed most easily, not all 

 fixing solutions that are capable of giving good nuclear and 

 chromosomal fixation have the same excellence. 



If one has interest in the study of cytoplasmic inclusions, as 

 mitochondria, yolk-spheres or oil-drops, one should use a fixa- 

 tive that preserves the inclusions in a condition that resembles 

 as closely as possible that found in the living. Inclusions, espe- 

 cially mitochondria, often demand special methods. Cell pig- 

 ments also require special treatment. 



