38 INTRODUCTION TO CYTOLOGY 



lowered the gelatin "sets" it becomes a hydrogel. This setting is a 

 reversible process: if the temperature is again raised the sol state is 

 resumed. On the contrary, egg albumen is a hydrosol at ordinary 

 temperatures and becomes a hydrogel when heated; in this case the 

 change is an actual coagulation and is not reversible. Many of the 

 colloids of the cell are of this non-reversible type. "The evidence that 

 in this colloidal condition the transition from liquid to solid, from 

 sol to gel, tends especially to pass into an indefinite series of gradations 

 gave a basis for the explanation of that mixture of the properties of solids 

 and liquids which has puzzled students of protoplasm" (Harper 1919). 

 Protoplasm is thus easily coagulated, not only by a high temperature, 

 but by a variety of chemical substances. The "fixation" of the cell 

 structures by the reagents employed in cytological technique is primarily 

 a coagulation phenomenon, and in the act of coagulation a substance, 

 especially one as complex as protoplasm, undergoes an alteration in 

 physical structure. Although such fixing fluids preserve very well the 

 general structure of the cell, the effects of coagulation should always be 

 borne in mind in interpreting finer details in preparations of fixed cells, 

 and in evaluating the results of those who have made special studies on 

 the ultimate structure of protoplasm. 



Microdissection. Much has been added to our knowledge of the 

 physical nature of protoplasm in recent years through microdissection. 

 Certain workers, notably Barber (1911, 1914), Kite (1913), Chambers 

 (1914, 1915, 1917, 1918), and Seifriz (1918, 1920) have developed a 

 technique (fully described by Barber 1914, and Chambers 1918) whereby 

 they have been able to dissect living cells under the high powers of the 

 microscope, thus opening a most promising field for investigation. Kite, 

 working on the cells of several plants and animals, found that protoplasm 

 exists in the form of sols and gels of varying consistency, that of plant 

 cells being as a rule more dilute and less rigid than that of animals. The 

 cytoplasm is commonly somewhat more viscous than is usually thought, 

 having the consistence of a "soft gel," while the nucleus may often be 

 surprisingly firm. (See Chapter IV.) 



Chambers (1917), who gives a convenient bibliography of the subject, 

 states that in the early germ cells and eggs of certain animals the proto- 

 plasm is in the sol state with a surface layer in the gel state, whereas 

 adult cells are usually gels. He further asserts that the surface gel is 

 readily regenerated after injury, a new gel film being formed over the 

 injured area. As regards the structure of protoplasm, he finds it to 

 consist of a hyaline fluid carrying microsomes and macrosomes, which 

 measure less than 1 M and from 2-4 M in diameter respectively. Upon dis- 

 organization the macrosomes, which are more sensitive to injury than 

 are the microsomes, swell and go into solution, while the hyaline fluid 

 flows out and mixes with water or coagulates, forming a reticular or 

 granular structure. 



