END OF VOL. I
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For detailed studies of protoplasm see Delage, Hérédité, 2nd ed. 1903; Henneguy, Leçons sur la Cellule, 1896; Verworn, General Physiology, English ed. 1899; Wilson, The Cell in Development and Inheritance, 2nd ed. 1900. All these books contain full bibliographies.
As we shall see later, it is by no means easy to separate sharply Protozoa and Protophyta, the lowest animals and the lowest plants; and therefore in our preliminary survey to designate lowly forms of life, not formed of the aggregation of differentiated cells, we shall employ the useful term "Protista," introduced by Haeckel to designate such beings at large, without reference to this difficult problem of separation into animals and plants (see also p. [35] f.).
The "micron," represented by the Greek letter µ, is 1⁄1000 mm., very nearly 1⁄25,000 of an inch, and is the unit of length commonly adopted for microscopic measurements.
A solid substratum is required, to which the lower surface adheres slightly: that movement is complicated by a sort of rolling over of the upper surface, constantly prolonging the front of the pseudopodium, while the material of the lower surface is brought up behind. H. S. Jennings, Contr. to the Study and Behaviour of the Lower Organisms, 1904, pt. vi. p. 129 f., "The Movements and Reactions of Amoeba."