The known facts are still too scanty to enable us to state precisely what a centrosome is in a morphological sense, either as regards its actual structure or its relation to other parts of the cell. In its simplest form (Fig. io8, A) the centrosome appears under the highest powers as nothing more than a single granule of extraordinary minuteness which stains intensely with iron-hematoxylin, and can scarcely be distinguished from the cyto-microsomes except for the fact that it lies at the focus of the astral rays. In this form it appears at the centre of the young sperm-aster in various animals — for example in the sea-urchin (Boveri), in Chetopterus (Mead), and in In almost all cases, however, the centrosome afterwards assumes a more complex structure and becomes surrounded by certain envelopes, the relation of which, on the one hand, to the centrosome and, on the other hand, to the astral rays have not yet been fully cleared up.
Boveri, whose observations have been confirmed by Brauer, Hacker, and others, described the centrosome in the cleavage-asters of Ascaris as a small sphere containing a minute central granule ; and Brauer's careful studies on the spermatogenesis of the same animal showed that both these structures are persistent and that division of the sphere is preceded by division of the granule (Fig. 107). The central granule is exactly like the simple centrosome of the sperm-aster as described above, but we do not yet know with certainty the genesis of the sphere surrounding it, and hence cannot state whether this is part of the centrosome proper or a part of the centrosphere surrounding it. The former view is adopted by Boveri, who suggests the word " centriole " for the central granule ; and, according to his observations on Ascaris and on sea-urchins, the simple centrosome of the original sperm-aster enlarges to form the sphere, while the centriole afterwards appears within it. In the case of Thalassema, however, Griffin's observations leave no doubt that the central granule persists in its original form from its first appearance in the sperm-aster through every stage of the cleavage-amphiaster, dividing during the early anaphase in each aster and giving rise to the centrosomes of the daughter-asters in which it again appears as a simple granule at the focus of the rays without a trace of surrounding envelopes (Fig. 73). In the cleavage-amphiaster it is surrounded by a somewhat vague, rounded mass (apparently representing the entire " centrosome " of Boveri and Brauer), which in turn lies in a reticulated centrosphere, from which the rays radiate. Both these structures disappear during the late anaphase, leaving only the central granule. Here, therefore, the true centrosome certainly corresponds to the central granule or centriole ; and all the surrounding structures belong to the centrosphere.
As soon as we look further we find apparent departures from this simple type of centrosome. In leucocytes Heidenhain finds at the centre of the centrosphere not one or two, but always three, and sometimes four, granules, which he conceives as centrosomes forming a central group or microcentrum. In the giant-cells of bone-marrow the central group consists of a very large number (a hundred or more) of such granules, each of which is again conceived as a " centrosome " (Fig. II, D). In the sea-urchin (Ecleinus) Boveri states that the original simple centrosome of the sperm-aster enlarges greatly to form a relatively large, well-defined sphere in which appear numerous granules (centrioles), which he would compare individually with the elements of Heidenhain's " central group."
I have given a somewhat similar account of the facts in Toxopneustes, describing the centrosphere as a reticulated mass derived from an original granule or centrosome at the focus of the rays,I and many other investigators have been unable to find a distinct body to be identified as a centrosome within the centrosphere. As far as the sea-urchins are concerned, there is, I think, good reason to doubt not only my own former conclusions, but also those of Boveri. Both vom Rath ('95, 2) and Hill ('95) find at the centre of the centrosphere in sea-urchins a distinct black granule (" centrosome "), which becomes double in the early anaphase precisely as in Thalassema. Moreover, Griffin's studies under my direction show that the minute single centrosome of Thalassema entirely loses its staining-power after certain reagents and only comes into view after other treatment.' I am now, therefore, inclined to believe that many if not all of the accounts asserting the absence of a minute central centrosome in the centrosphere are based on unsuitable methods, and that in most of such cases, if not in all, it is really present.
However this may be, it is now certainly known that the centrosome is in some cases a granule so small as to be almost indistinguishable from the microsomes ; that in this form it is able to organize the surrounding cytoplasm into the astral system ; and that in this form it may be handed on by division from cell to cell. It may well be that in some cases such a centrosome may multiply to form a central group, as in leucocytes and giant-cells ; that it may enlarge to form a granular or reticular sphere, as Boveri describes ; and that the individual granules within such a sphere do not have the value of centrosomes. Such secondary morphological modifications do not affect the physiological significance of the centrosome as a permanent cell-organ, but they have an important bearing on the question of its relation to the other constituents of the cell.
The latter question has not been definitely answered. Butschli, who has been followed by Erlanger, regards the centrosome as a small differentiated area in the general alveolar structure ; and he describes it in the sea-urchin as actually made up of a number of minute vesicles (Fig. 8, B). Burger CO suggested that the entire attraction-sphere and aster arise by a centripetal movement of microsomes to form a radiating group the centre of which (centrosome) is represented by a condensed mass of the ground-substance. Watase C93, '94) added the very interesting suggestion that the centrosome is itself nothing other than a microsome of the same morphological nature as those of the astral rays and the general thread-work, differing from them only in size and in its peculiar Despite the ambiguity of the word " microsome " Watases suggestion is full of interest, indicating as it does that the centrosome is morphologically comparable to other elementary bodies existing in the cytoplasmic structure, and which, minute though they are, may have specific chemical and physiological properties.