THE ARCHOPLASMIC STRUCTURES Asters and Spindle The asters and attraction-spheres have a special interest for the study of cell-organs ; for these are structures that may divide and persist from cell to cell or may lose their identity and reform in successive cell-generations, and we may here trace with the greatest clearness the origin of a cell-organ by differentiation out of the structural basis. Two sharply opposing views of these structures are now held. Boveri ('88, 2), who has been followed in a measure by Strasburger, maintains that the attraction-sphere of the resting cell is composed of a distinct substance, "archoplasm," consisting of granules or microsomes aggregated about the centrosome as the result of an attractive force exerted by the latter. From the material of the attraction-sphere arises the entire achromatic figure, including both the spindle-fibres and the astral rays, and these have nothing to do with the general reticulum of the cell. They grow out from the attraction-sphere into the reticulum as the roots of a plant grow into the soil, and at the close of mitosis are again withdrawn into the central mass, breaking up into granules meanwhile, so that each daughter-cell receives one-half of the entire archoplasmic material of the parent-cell. This material is, however, wholly distinct from that of the general reticulum, not, as many earlier observers have maintained, identical with it. Boveri was further inclined to believe that the individual granules or archoplasmic microsomes were " independent structures, not the nodal points of a general network," and that the archoplasmic rays arose by the arrangement of these granules in rows without loss of their individuality.' In a later paper on the sea-urchin ('95) this view is somewhat modified by the admission that in this case the archoplasm may not pre-exist as formed material, but that the rays and fibres may be a new formation, crystallizing, as it were, out of the protoplasm about the centrosome as a but having no organic relation with the general reticulum.
Strong evidence against the archoplasm-theory has been brought forward by many investigators, and I believe it to be in principle untenable. Nearly all recent workers have accepted in one form or another the early view of Biitschli, Klein, and Van Beneden that the astral rays and spindle-fibres, and hence the attraction-sphere, arise through a morphological rearrangement of the pre-existing protoplasmic network, under the influence of the centrosome. Although this view may be traced back to the early work of Fol (73) and Auerbach (74), it was first clearly formulated by Butschli (76), who regarded the aster as the optical expression of a peculiar physicochemical alteration of the protoplasm primarily caused by diffusion-currents converging to the central area of the asters An essentially similar view is maintained in Butschli's recent great work on the astral " rays " being regarded as nothing more than the meshes of an alveolar structure arranged radially about the centrosome (Fig. 8, B) . The fibrous appearance of the astral rays is an optical delusion, for they are not fibres, but flat lamellae forming the walls of elongated closed chambers. This view has more recently been urged by Reinke and Eismond.
The same general conception of the aster is adopted by most of those who accept the fibrillar or reticular theory of protoplasm, the astral rays and spindle-fibres being regarded as actual fibres forming part of the general network. One of the first to frame such a con
ception was Klein (78), who regarded the aster as due to " a radiar arrangement of what corresponds to the cell-substance," the latter being described as having a fibrillar The same view is advocated by Van Beneden in 1883. With Klein, Heitzman, and Frommann he accepted the view that the intra-nuclear and extranuclear networks were organically connected, and maintained that the spindle-fibres arose from "The star-like rays of the asters are nothing but local differentiations of the protoplasmic . . . In my opinion the appearance of the attraction-spheres, the polar corpuscle (centrosome) and the rays extending from it, including the achromatic fibrils of the spindle, are the result of the appearance in the egg-protoplasm of two centres of attraction comparable to two magnetic poles. This appearance leads to a regular arrangement of the reticulated protoplasmic fibrils and of the achromatic nuclear substance with relation to the centres, in the same way that a magnet produces the stellate arrangement of iron filings."' This view is further developed in Van Beneden's second paper, published jointly with Neyt ('87). "The spindle is nothing but a differentiated portion of the asters." 2 The aster is a "radial structure of the cell-protoplasm, whence results the image designated by the name of aster." 8 The operations of cell-division are carried out through the " contractility of the fibrillae of the cell-protoplasm and their arrangement in a kind of radial muscular system composed of antagonizing groups." 4 An essentially similar view of the achromatic figure has been advocated by many later workers. Numerous observers, such as Rabl, Flemming, Carnoy, Watase, Eismond, Reinke, etc., have observed that the astral fibres branch out peripherally into the general reticulum and become perfectly continuous with its meshes. This is very clearly shown in the formation of the sperm-aster about the middle-piece of the spermatozoon. In the sea-urchin (Toxopneustes) the formation of the rays from the cytoplasmic reticulum can be followed step by step, and there can, I think, be no doubt that the astral rays arise by a direct transformation or morphological rearrangement of the pre-existing structure, and that they extend themselves at their outer ends, as the sperm-aster moves through the egg-substance, by progressive differentiation out of this Once formed, however, the rays may possess a considerable degree of persistence and may actively elongate by growth. Only thus can we explain the pushing in of the nuclear membrane by the ingrowing spindle-fibres during the prophases of mitosis in certain forms (p. 5o) and the bending of the rays when two asters collide, as recently described by Kostanecki and Wierzejski ('96). It seems certain, furthermore, that during the rotation of the amphiaster in the formation of the polar bodies (Fig. 71) and in similar cases, the spindle, at least, moves bodily. The substance of the spindle or of the asters may, moreover, persist in the resting cell, after the close of mitosis, as the attractionsphere or paranucleus (Nebenkern), and in such cases the term " archoplasm " may conveniently be retained for descriptive purposes. To regard the archoplasm as a primary and independent constituent of the cell would, however, as I believe, be an error.