n PRACTICAL DIRECTIONS 259 



or excluded from either. When any given " protist " 

 has to be classified the case must be decided on its 

 individual merits : the organism must be compared in 

 detail with all those which resemble it closely in structure, 

 physiology, and life-history ; and then a balance must 

 be struck and the doubtful form placed in the kingdom 

 with which it has, on the whole, most points in common. 

 It will no doubt occur to the reader that, on the 

 theory of evolution (p. 222), we may account for the 

 fact of the animal and vegetable kingdoms being related 

 to one another like two trees united at the root, by the 

 hypothesis that the earliest organisms were protists, 

 and that from, them animals and plants were evolved 

 along divergent lines of descent. ^ 



PRACTICAL DIRECTIONS 



Sphaerella. Examine a drop of water containing Sphaer- 

 ellae (see note on p. 238), first with the low power, and then, 

 after putting on a cover-glass, with the high power. Note 



1. Their rounded form and green (or red) colour \ the 

 cellulose cell-wall ; the protoplasm, enclosing (a) a chroma- 

 top hore containing chlorophyll (the red colour is due to 

 another colouring matter, hcematochrome], and (b) a central 

 nucleus, seen better later ; and in the active forms, the 

 twoflagella. Sketch. 



2. Dissolve out the chlorophyll by adding alcohol ; the 

 nucleus will then be visible, and may be made more distinct 

 by staining with magenta, or a solution of potassium iodide 

 in water saturated with iodine. After treatment with the 

 iodine solution, a bluish coloration will often be seen around 

 the small starch-containing pyrenoids. 



3. Treat some specimens either with strong iodine 

 solution and then with sulphuric acid (75 per cent.), or with 

 Schulze's solution, which is prepared as follows : make 

 about 25 c.c. of a syrupy solution of zinc chloride, and let 

 it stand for a short time ; then pour off the clear fluid, 

 saturate it with potassuim iodide and add iodine until the 

 solution has about the colour of sherry. Note the blue 

 coloration of the cell-wall. 



4. In the living specimens n6te also the mode of division 



S 2 



