42 



ESSENTIALS OF ZOOLOGY 



It is in a colloidal state of the emulsoid type. In the emulsoid, or 

 colloidal emulsion, the substances are distributed through the more 

 watery or dispersion medium. A colloid is identified by the presence 

 of particles which are groups of molecules dispersed through a more 

 fluid or Avatery phase. These particles, of course, are larger than 

 molecules, but they are too small to be seen with the ordinary 

 microscope. It is possible for water and substances in solution to 

 enter protoplasm from without, and this is reversible. With loss 

 of water from the dispersion medium the dispersed particles of the 

 colloid become congested by loss of general fluidity. This condition 

 is known as the gel state. When there is increased water in the 

 dispersion medium and the particles move with greater ease in the 

 more fluid medium, the colloidal state tends to become sol. This 

 • transfer of water may be due to chemical changes in the dispersed 

 particles or in the dispersion medium of the colloid. The ability 



Fig 10 The change of a colloidal emulsion from sol to gel state. In A the drop- 

 lets of the disperse phase (not stippled) are shown scattered through the disper- 

 sion medium (stippled), and the emulsion is a sol; in B the droplets are shown 

 taking up liquid and swelling; in C this is continued until they press upon one 

 another- in D the droplets are so crowded as to become contmuous and to have be- 

 come in' fact the dispersion medium, while that which was the dispersion medium 

 is now in droplets and has become the disperse phase. The emulsion has become a 

 gel. (From Wolcott, Animal Biology, McGraw-Hill Book Company, Inc.) 



of protoplasm, because of its colloidal nature, to change from sol 

 to gel state and back to sol repeatedly is the basis of many of the 

 vital activities, such as utilization of food, disposal of waste, and 

 movement. 



Fundamental Properties or Activities of Protoplasm 



In addition to the general characteristics, there may be mentioned 

 and described briefly a number of important activities common to 

 all protoplasm. These properties are : 



1. Irritahility, which refers to the capacity present in all proto- 

 plasm for responding to changes in environmental conditions, or ex- 

 ternal stimuli. 



