[C)6 FINE-STRUCTURE OF PROTOPLASM 



II 



cytoplasm. Nor can the polarity of Tithifex eggs be reversed by 

 centrifuging (Lehmann, 1940). 



We must mention in particular that neither the oil droplets nor the 

 yolk and pigment combine into a homogeneous phase, but remain 

 dispersed. This indicates the existence of surface layers which, either 

 by their structure or by their electric charge, offer resistance to fusion 

 with the neighbouring particles. It is quite possible that the properties 

 of the ground substance in which they are still embedded prevent the 

 droplets from clustering together as might be expected from the laws 

 of surface tension. 



Separation of phases as a result of freezing. When the cytoplasm is 

 subjected to freezing, ice crystals are formed which are embedded in 

 the dehydrated gel. Thus we get separation by crystalHzation. Ac- 

 cording to LuYET (1939) the dehydration of the living hydrogel 

 proceeds step by step. As long as the freezing is confined to excess 

 water, such as that contained in the vacuoles of plant cells or coming 

 from the metaboHc process, the cell does not die. It is only when the 

 imbibition water which takes up the plasma structure is withdrawn 

 from the living hydrogel, that the structure breaks down and death 

 of the cell sets in. The resistance of the cytoplasm to low temperature 

 depends, therefore, on the persistence with which it retains its hy- 

 dration water and safeguards it against crystalHzation. 



Thecrystalhzationof the imbibition water, which is enclosed in the 

 submicroscopic gel meshes and bound by hydration forces, can be 

 prevented if the gel is cooled down to very low temperatures by rapid 

 removal of heat. This leads to a state which has been designated as 

 vitrification (Luyet, 1937). The water molecules become immobile to 

 such an extent that they cannot arrange themselves into a crystal 

 lattice and retain their original positions with respect to the sub- 

 microscopic gel strands. In this way it is possible to preserve the "life 

 structure" of thin protoplasmic films for a considerable space of time, 

 for instance in hquid air. The fact, however, that with rising tempera- 

 ture the preparation has to pass through the critical temperature range 

 in which the water separates from the gel by crystallization, makes it 

 difficult to induce such a "vitrified" protoplasm to resume its life 

 functions. The clear gel suddenly becomes turbid at about — 15''^ 

 and then the structural breakdown sets in, which normally causes 

 death on slow cooling (cf. freeze-drying, p. 178). 



