The genesis of the reticular type of centrosphere is not so well determined. In Nereis the aster (maturation-asters, sperm-aster) has at first nothing more than a minute centrosome at its centre. This becomes surrounded at a later period by a large reticulated centrosphere, showing no sign of radial arrangement, that appears to arise by a transformation of the inner ends of the astral rays. A nearly similar account is given by Hill in the case of the spermaster in Spluerechinus and Phallusia. In these latter cases the centrosphere shows no differentiation into cortical and medullary zones. In Thalassema and Nereis, on the other hand, the minute centrosome becomes surrounded by a somewhat vague body distinctly different from the reticulum of the outer centrosphere, and this body perhaps represents a " medullary zone." This body, with the centrosome, corresponds very nearly to the " centrosome " of Ascaris with its " centriole " or central granule as described by Boveri and Brauer ; but in Thalassema Griffin's observations show conclusively that the minute central granule alone is the centrosome, and that the surrounding body does not persist after division. I cannot avoid the suspicion that the body described by Boveri as the " centrosome " in Echinus may represent this medullary region of the centrosphere, and that he, like myself, may have overlooked the centrosome. Nor does it seem impossible that the " centriole " or central granule of Ascaris (Boveri, Brauer) may likewise represent the true centrosome. These questions can only be cleared up by further investigation.
To sum up : The history of the " archoplasmic " structures gives strong ground for the conclusion that attraction-spheres, asters, and spindle are, like the nucleus, differentiations of the general cell-network, which is, as it were, moulded by the centrosome into a specific form. If this be well founded, the word " archoplasm " has no significance save in a topographical or descriptive sense. In this light it is an interesting fact that the aster or attraction-sphere may either persist and divide, like a permanent or may disappear and re-form in successive cell-generations.
A minute analysis of the various parts of the cell leads to the conclusion that all cell-organs, whether temporary or " permanent," are local differentiations of a common structural basis. Temporary organs, such as cilia or pseudopodia, are formed out of this basis, persist for a time, and finally merge their identity in the common basis again. Permanent organs, such as the nucleus or centrosome, are constant areas in the same basis, which never are formed de novo, but arise by the division of pre-existing areas of the same kind. These two extremes are, however, connected by various intermediate gradations, examples of which are the contractile vacuoles of Protozoa, which belong to the category of temporary organs, yet in many cases are handed on from one cell to another by fission, and the attraction-spheres and asters, which may either persist from cell to cell or disappear and re-form about the centrosome.
The facts point strongly to the conclusion, which has been especially urged by De Vries and Wiesner, that in many if not in all cases the division of cell-organs is in the last analysis brought about by the division of more elementary masses of which they are made up ; and furthermore that the degree of permanence depends on the degree of cohesion manifested by these masses. The clearest evi
dence in this direction is afforded by the chromatic substance of the nucleus, the division of which does not take place as a single massdivision, but through the fission of more elementary discrete bodies of which it consists or into which it is resolved before division. Several orders of such bodies are visible in the dividing nucleus, forming a series of which the highest term is the plurivalent chromosome, the lowest the smallest visible dividing basichromatin-grains, while the intermediate terms are formed by the successive aggregation of these to form the chromomeres of which the dividing chromosomes consist. Whether any or all of these bodies arc "individuals " is a question of words. The facts point, however, to the conclusion that at the bottom of the series there must be masses that cannot be further split up without loss of their characteristic properties, and which form the elementary morphological units of the nucleus.
There is reason to believe that the linin-network is likewise composed of minute bodies, the oxychromatin-granules, which are closely similar in appearance to the smallest chromatin-grains, and differ from them only in chemical nature as shown by the difference of staining-power. Whether the oxychromatin-granules have also the power of growth and division is unknown ; but if, as Van Beneden and Heidenhain maintain, the basichromatin- and oxychromatin-granules be only different modifications of the same element, a presumption certainly exists that they have such powers. When we extend this comparison to the cytoplasm, the ground becomes more uncertain. It seems well established that the cytoreticulum is of the same nature as the linin-network. If this be admitted, we are led to accept on a priori grounds that some at least of the cytomicrosomes are not artefacts, but morphological bodies comparable with those of the linin and chromatin networks, and like them capable of growth and division. This conclusion is, as yet, no more than a somewhat doubtful inference. In the centrosome, however, we have a body, no larger in many cases than a " microsome," which is positively known to be a persistent morphological element, having the power of growth, division, and persistence in the daughter-cells. Probably these powers of the centrosome would never have been discovered were it not that its staining-capacity renders it conspicuous and its position at the focus of the astral rays isolates it for observation. When we consider the analogy between the centrosome and the basichromatin-grains, when we recall the evidence that the latter graduate into the oxychromatin-granules, and these in turn into the cytomicrosomes, we must admit that Brucke's cautious suggestion that the whole cell might be a congeries of self-propagating units of a lower order is to-day not entirely without the support of facts.