536 THE NATURAL HISTORY OF 



greater part of the primordial cell, which then appears 

 as an almost entirely colourless, sharply-defined globule, 

 owing to the fact that the protoplasm is invariably sur- 

 rounded, where it is free, by a sharp border, as if by a 

 membrane. It is only in the middle or at one end of 

 the globule that there generally remains a deposit of the 

 green substance in the form of a ring or lateral mass, 

 (Fig. 26.) 



The same colourless protoplasm occurs in all cells, 

 even where the other coloured substances are much 

 developed ; especially does it always appear as a delicate, 

 almost imperceptible layer constituting the outer boundary 

 of the coloured primordial cell, the periphery of which 

 then becomes sharply defined, and as it were surrounded 

 by a delicate, transparent, membrane. (Fig. 16.) Besides 

 this, the colourless protoplasm seems to occur exclusively 

 at the anterior extremity of the primordial cell, where it is 

 produced into a conical elongation or beak. (Figs. 18, 36.) 



The green substance appears sometimes as a thin and 

 fluid, sometimes gelatinous, sometimes more solid mucus, 

 and is perhaps more abundant and better developed in 

 the motile than in the still form. 



The red substance generally forms only a central mass 

 of greater or less size ; more rarely it constitutes exclu- 

 sively the contents of the primordial cell. It is interesting 

 to trace all the stages of the transition from the green 

 into the red substance, and one stage or phase especially 

 has long been regarded with great interest, in which 

 the red pigment is reduced to a single minute granule, 

 attached to the interior or to one side of the primordial 

 cell, then representing what is described by Ehrenberg as 

 the "red eye spot" of the Infusoria, and which was dis- 

 covered by Kiitzing, Fresenius, and Thuret, in the spores 

 of Algae. 



The three substances just considered present themselves 

 in the form of colourless, red, and green globules, 

 granules, and drops; but besides these, the primordial 

 cells contain, at times, vacuoles, chlorophyll- vesicles, 



