CELLS AND TISSUES. 31 



centrated salt solution is required to precipitate gelatine 

 at a higher temperature than at a lower one. Corre- 

 sponding with this, it is an easy matter to prepare con- 

 centrations in which a precipitation will not take place 

 until a temperature below that at which gelatine becomes 

 solid has been reached. When this temperature has 

 been reached a precipitate is produced, this time also in 

 the form of fine droplets, in the solid and originally 

 entirely clear gelatine. We can therefore not regard 

 such a separation of droplets as an undeniable proof 

 of the fluid character of a medium. That, however, the 

 application of the theory of surface tension to certain 

 cellular phenomena may be of great service, as shown 

 by the observations of ALBRECHT, is of course not ques- 

 tioned by the above experiment. Nor does anything stand 

 in the way of looking upon the shrinkage forms of thin 

 solidified gelatine (BuxscHLi, PAULI) in alcohol, am- 

 monium sulphate solution, etc., as expressions of sur- 

 face tension. In fact, the similarity is very great between 

 such shrinkage forms .and the shapes of suspended 

 ("schwerloser") masses of oil (cubes, cylinders, etc.) in 

 suitable media, as described by PLATEAU. The diffi- 

 culties that are encountered in endeavoring to explain, 

 on the basis of the solid nature of protoplasm, the 

 unhindered appearance and solution of crystals without 

 the formation of holes, do not exist in the case of 

 gels, as experiment has taught. LUDEKING has been 

 able to demonstrate with the polarization microscope the 

 appearance of ice crystals in the clear substance of thin 

 slices of deeply cooled ( i8C.) gelatine. One can 

 also notice in salt-gelatines in which the crystalloid, such 

 as ammonium chloride, shows a great fall in solubility 



