Biologische Studien
Authors: Haeckel Ernst 
Published: April 2021

ABSTRACT
The volume which Dr. Haeckel has just published contains reprints of several papers already put out by him in the ‘Jenaische Zeitschrift,’ together with two additional notices of new forms of Monera which we have not previously seen. The monograph of the Monera is already well known to our readers, since it was reproduced in full in this Journal during the year 1869. Succeeding this in the present volume is a series of papers entitled “Contributions to the Plastid Theory.” In the first of these Haeckel points out what he means by the ‘Plastid theory,’ which he discusses in connection with the generally received ‘Cell theory.’ Just as at one time the cell was conceived to be the simplest living form, as seen in the ovum and unicellular organisms, and just as it was conceived that organisms are built up by aggregations of these simplest morphological units, so must we now admit the existence of still simpler units—the simplest conceivable—mere bits of protoplasm, undifferentiated, without nucleus, living freely as Monera, and possibly also becoming aggregated also to form tissue. That such units should exist is what we were gradually led to expect by the researches of Max Schultze and others, resulting in the abandoning of the cell-wall, and the rise of the all-important protoplasm-theory. It is Haeckel who has discovered them. He calls these simplest units Cytods, and classes Cytods and Cells together under the head Plastids. The cytod being the simplest possible form of life, it is this form under which life first appeared, and it is this which we should look to see formed by so-called spontaneous generation. In the course of development the cytod has given rise to the cell by internal differentiation of a nucleus. Since the development of the individual (Ontogeny) is a more or less complete epitome of the development of the species (Phylogeny), we should expect such Plastids as are cells to pass through the cytod condition in their life-cycle, and we find that they actually do. Since the cytod is the earliest form, all organic beings have sprung from it, but all but Monera have passed through the cell stage also, and hence in all the higher forms the cytod condition in the development of the individual is obscure, the ovum appearing first as a cell, though Amœbæ, Gregarinæ, and Radiolaria reproduce by cytods. But we may add to what Dr. Haeckel says on this point, that if we trace the development of the ovarian ovum, we actually can and do follow it back to the cytod condition. Dr. Haeckel would suggest that the disappearance of the nucleus which certainly occurs in some ova, at the time of impregnation (though in others it as certainly persists), is a reversion to the ancestral cytod form and is to be explained in this way. These views give very great importance to the cell-nucleus, and it is well to dwell on this, since there is a tendency to overlook it, even to consider it an artificial product of the reagents used in microscopical investigation. Dr. Haeckel is very firm on this question; he points to a division of labour between nucleus and surrounding plasm, and adduces the observation of the nuclei in the cells of living transparent pelagic organisms to confront those who doubt their living existence. On the other hand, he distinguishes two kinds of cytods. It is not every Plastid devoid of nucleus which is a cytod, for by “degradation’’ or “retrogressive metamorphosis” (Rückbildung), a cell may lose its nucleus- and is not then to be confounded with an ancestral, nucleusless cytod, though like it in simplicity. It is a “shamcytod” or “dyscytod.” Such are the red blood-cells and the horny epidermic scales of mammals. Plastids and cytods have been elsewhere further classified by Haeckel according to such characters as the presence of a wall (cell-wall), &c. It is this modification, then, of the cell-theory, viz., the Plastidtheory, which gives the Monera so much interest in connection with the doctrines of evolution and the question of Abiogenesis.