REJUVENESCENCE IN NATURE. 207 



commencement of the division of the contents by vi^hich 

 the latter are formed, a change begins in the colour of 

 the parent-cell, the red colour retreating to some extent 

 from the periphery, and a yellow (sometimes rather 

 greenish) border forming round the deep red inner mass. 

 The young swarmers also, for a short time after they 

 issue out, have only a narrow yellow rim round a dark 

 red middle. During the two to three days' period of 

 movement and growth of these swarming- cells, — in which 

 they grow to about four times the original size, changing 

 their obtusely ovate form, at the same time, to a re- 

 versed pear-shaped, apiculated shape, and forming a 

 delicate enclosing membrane (p. 158), — important new 

 changes take place in the contents of the cells. The red 

 colour becomes more and more concentrated into the 

 middle of the cell, so that a sharply defined bright red 

 nucleus is formed, in the interior of which a lighter space 

 is often clearly perceptible, corresponding to the nuclear 

 vesicle above mentioned, around which the red colouring 

 matter forms a covering, mostly complete, but sometimes 

 imperfect and interrupted. The rest of the cell-contents 

 have become a brilliant green, and in them may be 

 clearly distinguished the above-mentioned starch-globules, 

 as well as many more smaller green granules. The 

 ciliated point of the cell, often drawn out like a beak, is 

 colourless. This first moving generation is succeeded by 

 a not yet accurately determined number of similar active 

 generations, populating the water for some weeks, and 

 often giving it a bright green colour, till at length 

 universal rest recommences, and the cells sink to the 

 bottom or attach themselves to the sides. The transition 

 from one active generation to another takes place through 

 a transitory resting generation of extremely short duration. 

 The full-grown swarming-cells finally come to rest within 

 their wide shirt-like envelope, and almost simultaneously 

 divide into two cells, which, without becoming active, 

 divide again into two cells. Thus, within the mother- 

 envelope are produced four daughter-cells (more properly 



