Ixiv THE ANIMAL KINGDOM. 



and continued by Gerstaecker, Hoffmann, Giebel, Hubrecht, Vosmaer, Blitschli, and 

 others, is a fitting embodiment of the results of higher zoological studies from Lin- 

 naeus to the present time. 



3. Period of Morphology and Embryology. — This period has been distinguished 

 (1) by the application of the discovery by Schwann of the cellular theory of organized 

 beings, especially to animals, and the studies of Dujardin and W. Schultze on the nature 

 of protoplasm, proving that the cell is the unit of organization, and that protoplasm 

 is the basis of life ; (2) by the application of histological discoveries and methods to 

 embryological research, and (3) by the application of the doctrine of evolution as a 

 working theory to account for the common origin of animals from a single simple 

 organism. If single names are to be mentioned where in fact many have worked 

 together to accomplish these results, the names of Schwaim, of Dujardin and 

 Schultze, and that of Darwin come first to mind. 



In 1665 Robert Hooke distinguished the cells of plants, calling them " cells and 

 pores," and comparing them to honey-comb. Schwann was the first to discover 

 animal cells. Schwann first (1839) called the nucleus ' korperchen,' but Valentine 

 in the same year (1839) invented the terms nucleus and nucleolus, since then in uni- 

 versal use, and Valentine was the first, in his review of Schwann's work in the 

 Repertorium for 1839, to speak of 'the cellular theory.' 



In Carnoy's La Biologie Cellulaire (1884) we find a convenient summary of the 

 history of the discovery of protoplasm and the doctrine that it forms the living matter 

 common to vegetable and animal cells. In 1835 Dujardin thus characterized this 

 substance : " I propose to name sarcode that which other observers have called a 

 living jelly, this glutinous, transparent, homogeneous substance, which refracts light 

 a little more readily than water, but much less than oil; which is extensible and can 

 stretch itself like mucus, is elastic and contractile, is susceptible of spontaneously 

 forming spherical cavities or vacuoles, filled with the surrounding liquid, which some- 

 times forms of it an open network. . . . Sarcode is insoluble in water ; at length, 

 however, it ends by decomposing and leaving behind a granulous residue. Potassium 

 does not suddenly dissolve it, as it does mucus or albumen, and appears only to hasten 

 its decomposition by water ; nitric acid and alcohol suddenly coagulate it and render 

 it white and opaque. Its properties are then very distinct from those of substances 

 with which some authors have confounded it, for its insolubility in water distinguishes 

 it from albumen, and its insolubility in potash likewise distinguishes it from mucus, 

 gelatine, etc. . . . The most simple animals, amoebas, monads, etc., are wholly com- 

 posed, at least in appearance, of this living jelly. In the higher Infusoria it is con- 

 tained in a loose tegument which opens on its surface like a network, and through 

 which it can pass out in a state of almost perfect isolation. . . . We find sarcode in 

 eggs, zoophytes, worms and other animals; but in these it is susceptible of receiving 

 with age a degree of organization more complex than in animals lower in the scale. 

 . . . Sarcode is without visible organs, and without appearance of cellulosity; but, 

 however, it is organized, because it throws out divers prolongations, drawing along in 

 them granules, alternately extending and retracting them, and, in a word, it has life." 



The observations of the past fifty years have made little change in Dujardin's 

 characterization of this substance, but his name has become well nigh forgotten, and 

 for it has been substituted the word protoplasm. This new word was first bestowed 

 upon it by Purkinje in 1839-40. Afterwards the celebrated botanist Hugo von Mohl, 

 ignorant of the existence of the word, said in 1846 (Bot. Zeitung), "I believe myself 



