NATURAL RHYTHM. 171 
specimens of Aurelia into a solid block of ice. Of 
course, as sea-water had to be employed, the cold 
required was very considerable ; but I succeeded in 
turning out the Medusze encased on all sides in a 
continuous block of sea-water. By now immersing 
this block in warm water, I was able to release the 
contained specimens, which then presented a very 
extraordinary appearance. The thick and massive 
gelatinous bell of a Medusa is, as every one knows, 
chiefly composed of sea-water, which everywhere 
enters very intimately into the structure of the 
tissue. Now, all this sea-water was, of course, 
frozen in situ, so that the animals were every- 
where and in all directions pierced through by an 
innumerable multitude of ice-crystals, which formed 
a very beautiful meshwork, pervading the whole 
substance of their transparent tissues. 
These experiments were made in order to ascertain 
whether the Meduse, after having been thus com- 
pletely frozen, would survive on being again thawed 
out, and, if so, whether the freezing process would 
exert any permanent influence on the rate of their 
rhythm. Now in all the cases the Medusz, after 
having been thawed out, presented a ragged ap- 
pearance, which was due to the disintegrating 
effect exerted by the ice-crystals while forming in 
the tissues; yet notwithstanding this mechanical 
injury superimposed on the physiological effects of 
such extreme cold, all the Medusz recovered on 
being restored to sea-water of the normal tem- 
perature. The time occupied by the process of 
recovery varied in different individuals from a few 
