CHROMATIN, LININ, AND THE CYTORETICULUM What, now, is the relation of the smallest visible chromatin-grains to the linin-network and the cytoreticulum ? Van Beneden long ago maintained I that the achromatic network, the nuclear membrane, and the cytoreticulum have essentially the same structure, all consisting of microsomes united by connective substance, and being only " parts of one and the same structure." But, more than this, he asserted that the chromatic and achromatic microsomes might be transformed into one another, and were therefore of essentially the same morphological nature. "They pass successively, in the course of the nuclear evolution, through a chromatic or an achromatic stage, according as they imbibe or give off the chromophilous substance." 2 Both these conclusions are borne out by recent researches. Heidenhain ('93, '94), confirmed by Reinke and Schloter, finds that the nuclear network contains granules of two kinds differing in their staining-capacity. The first are the basichromatin granules, which stain with the true nuclear .dyes (basic anilines, etc.), and are identical with the "chromatin-granules " of other authors. The second are the oxychromatin-granules of the linin-network, which stain with the plasma-stains (acid anilines, etc.), and are closely similar to those of the cytoreticulum. These two forms graduate into one another, and are conjectured to be different phases of the same elements. This conception is furthermore supported by many observations on the behaviour of the nuclear network as a whole. The chromatic substance is known to undergo very great changes in staining-capacity at different periods in the life of the nucleus (p. 244), and is known to vary greatly in bulk. In certain cases a very large amount of the original chromatic network is cast out of the nucleus at the time of the division, and is converted into cytoplasm. And, finally, in studying mitosis in seaurchin eggs I was forced to the conclusion ('95, 2) that a considerable part of the linin-network, from which the spindle-fibres are formed, is actually derived from the chromatin.
When all these facts are placed in connection, we find it difficult to escape the conclusion that no definite line can be drawn between the cytoplasmic microsomes at one extreme and the chromatin-granules at the other. And inasmuch as the latter are certainly capable of growth and division, we cannot deny the possibility that the former may have like powers. It may well be that our present reagents do not give us a true picture of these elementary units —that " microsomes " are but a rude semblance of reality. That they are nevertheless an expression of the morphological aggregation of the protoplasmic network out of more elementary units, must, I think, be accepted as a working hypothesis. Whether they are elementary organisms in Altmann's sense, whether they have a persistent morphological identity, whether they arise solely by the division of preexisting microsomes, or may undergo dissolution and reformation, whether, in short, they are the self-propagating elementary bodies postulated by so many eminent naturalists as the essential basis of the cell, — all these are entirely open questions which the cytology of the future has to solve.