Page:The Scientific Monthly vol. 3.djvu/303

 ORIGIN AND EVOLUTION OF LIFE 297

Evolution of Protoplasm and Chromatin, the Two Structural

Components op the Living World

It is still a matter of controversy^® whether any bacteria, even at the present time, have reached the evolutionary stage of the typical cell with its cell wall, its contained protoplasm and its distinct nuclear form and inner substance known as chromatin. Some bacteriologists (Fischer) maintain that bacteria have neither nucleus nor chromatin; others admit the presence of chromatin but deny the existence of a formal nucleus; others contend that the entire bacterial cell has a chromatin content; while still others claim the presence of a distinctly differentiated nucleus containing chromatin. Most of them, however, are agreed as to the presence in bacteria of granules of a chromatin nature, while they leave as an open question the presence or absence of a structurally distinct nucleus. This conservative point of view is borne out by the fact that all the common bacteria have been found to contain nuclein, the specific nuclear protein complex; and is also sustained by the fact that the lowliest plants, the blue-green algae (Cyanophyceae), contain neither definitely formed nuclei nor chromatin bodies,^® and are thus regarded as intermediate between bacteria and the higher green algae (Chlorophyceae).

It is also a matter of controversy among bacteriologists as to the veiy important question whether protoplasm or chromatin is the more ancient. Cell observers (Boveri, Wilson, Minchin), however, are thor- oughly agreed on this point. Thus Minchin is unable to accept any theory of the evolution of the earliest forms of living beings which assumes the existence of forms of life composed entirely of protoplasm without chromatin.^** All the results of modern investigations — the combined results, that is to say, of cytology and protistology — appear to him to indicate that the chromatin-elements represent the primary and original living units or individuals, and that the protoplasm repre- sents a secondary product. As to whether chromatin or protoplasm is the more ancient, Boveri suggests that true cells arose through sym- biosis between protoplasm and chromatin, and that the chromatin elements were primitively independent, Kving symbiotically with pro- toplasm. The more probable view is that of Wilson, that chromatin and protoplasm are coexistent in cells from the earhest known stages, in the bacteria and even probably in the ultra-microscopic forms.

The development of the cell theory after its enunciation in 1838 by Schleiden and Schwann followed first the differentiation of proto- plasmic structure in the cellular tissues (histology). Since 1880 it has taken a new direction in investigating the chemical and functional

18 1. J. Kligler.

!• Loeb, Jacques, 1906, p. 106.

20 Minchin, E. A., 1916, p. 32.

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