it and ensheathe it, and form groups in the nuclear net. That which clings to the chromosomes become modified to form the "Halbspindel fasern." As the spireme splits, nucleolar substance still persists around the daughter spiremes. This tends to collect at the middle of the curved chromosomes which are turned with the convexity towards the centrosphere. As the daughter spiremes separate, nucleolar substance unites with the linin to form the rays of the central spindle. The linin in this phase stains slightly deeper than the cytoplasmic reticulum but not so deeply as does the nucleolar substance. It is only in the telophase stage that the nucleolar substance, which has been associated with the linin, becomes indistinguishable from it.

There seems to be conclusive evidence, therefore, that the nucleolus of the adult nerve cell is a heterogeneous structure with an oxyphile center which is structurally and chemically allied to the linin, and with a peripheral zone of basophile chromatin.

The Centrosome.

HOLMGREN finds that the centrosome in the spinal ganglion cell of Lophius is located in the center of the cell. It forms the center of the concentric circles and radial "Züge" of the cytoplasm. HOLMGREN suggests that, since its reproductive function must have ceased, and since it holds this constant relation to the trigroid substance, it is probably concerned in the nutritive functions of the metabolism of the cell. The object which LENHOSSÉK first interpreted as the centrosome in the spinal ganglion cell of Rana is, according to HOLMGREN, nothing other than a section through an invaginated trabecula of the capsule.

KOLSTER identifies the centrosome in the nerve cells of Petromyzon even in unstained preparations, but it is not located in the center of the cell. The centrosphere is surrounded by an irregularly shaped mass of granules which are arranged in the form of a dense reticulum, which we have already discussed in connection with the ground substance of the cytoplasm. The interior of the body in unstained preparations appears as dark, circular lines separated by bright points. In other methods, it appears as a circular space in which the centrosome lies.

The centrosome in the nerve cell of the dog and rabbit infected with rabies has been studied by NELIS. He holds that the organ is not visible in the normal nerve cells of mammals, but that it is brought into plain view during chromatolysis which accompanies rabies. He suggests that this reappearance of the centrosome immediately before the nucleus atrophies indicates a tendency toward cell division at that

time. Such a hypothesis is supported also by the fact, observed by several investigators, that karyokinetic figures occur in the nerve cells. of animals infected with rabies.

HATAI, however, finds the centrosome present in the nerve cell of the adult rat, though it is not so generally found in the adult as in The centrosphere is here densely surrounded with neurosomes which go out in radial lines from it, somewhat as KOLSTER finds in Petromyzon. Within the clear centrosphere HATAI finds, also, radii of extremely fine granules centering in the centrosome and staining like it. In some cases the place of the single centrosome may be taken by a number of smaller granules, but in such cases the radial arrangement in the centrosphere is lost. HATAI (02) describes also the behavior of the centrosome in the mitosis in the embryonic nerve cell. One of the two centrosomes seems to migrate to the opposite pole of the nucleus and then each divides, giving two centrosomes in each centrosphere during the mitotic process.

I.

The Tigroid Substance and Chromatolysis.

Structural and Chemical Features.-The studies of SCOTT ('99) upon the chemistry of the tigroid substance of the nerve cell are especially noteworthy since they are based upon both embryological and comparative methods, and since he has employed both cytological and micro-chemical technique. He has not confined himself to the use of the NISSL method, but has used toluidin blue and eosin to differentiate the oxyphile and the basophile substances.

By the haematoxylin method for the Prussian blue reaction after treatment of the tissue with acid ferrocyanide, and by the ferrous sulphide reaction after treatment with ammonium sulphide and glycerine, the basophile and oxyphile nuclear substance and the tigroid bodies show the presence of iron. By the oxide of molybdenum reaction after treatment with a nitric acid solution of ammonium molybdate followed by phenylhydrazin hydrochloride the same elements of the cell show the presence of phosphorus. In digestive tests Scott finds that immersion for several days in 0.2 per cent hydrochloric acid, at 37°C., does not affect the oxyphile nuclear substance, but after digestion in pepsin and hydrochloric acid the oxyphile substance cannot be demonstrated by the most vigorous stains. In the pepsin experiments the nucleolus sometimes disappears, but SCOTT considers. that it is only loosened from the slide by the digestion of its oxyphile center by which it may have been attached.

While treatment of the nerve cell with acid alcohol aids in dem

onstrating the presence of iron, prolonged treatment will extract all the iron and render the tissue colorless under the toluidin blue stain. Yet after all the iron has been extracted phosphorus is still demonstrable in the tigroid substance and in the oxyphile and basophile nuclear substance. Treatment with alkalies removes the iron from the tigroid substance but not from the nucleolus and the oxyphile nuclear substance, excepting when the treatment is prolonged, while the nuclei of neuroglia cells in the same preparations stain normally in eosin and toluidin blue. Prolonged treatment in the alkalies, however, does not affect the phosphorus of the cell. The author finds that fresh defibrinated ox-blood, owing to its alkalinity, has the same effect as potash of soda upon the nerve cells fixed in alcohol.

By these differential methods SCOTT concludes that the tigroid substance is one of at least three nuclein compounds found in the nerve cell, the other two being the "basophile covering of the nucleolus and the oxyphile nuclear substance." The descent of the three compounds was traced by SCOTT from the chromatin of the germinal cell. "The chromatin," he says, "divides into two parts, each containing iron and phosphorus, but the one is oxyphile and remains in the nucleus, while the other is basophile and diffuses into the cell body and becomes the NISSL granules." This position would seem to be supported, also, by VAN GEHUCHTEN (97) who is inclined to think that the nuclein (chromatin) of the karyochrome cell is equivalent to the chromatin substance of the somatochrome cell.

If the nuclear chromatin is the source of the first chromatic elements in the cytoplasm of the embryonic nerve cell, does it continue this role in the adult cell? If it does, in what manner is the distribution of the chromatic elements from the nucleus to the cytoplasm brought about?

HOLMGREN ('99) has described an elaborate process, which we have discussed in the section relating to the nucleus, by which the nuclear wall breaks down and the oxyphile nuclear substance migrates into the cytoplasm and becomes at the same time metamorphosed into basophile tigroid substance. By the same process also the basophile covering of the nucleolus breaks off in large masses which are carried out into the cytoplasm. SCOTT, however, since he considers that the breaking down of the nuclear membrane and the displacement of the nucleolus is an artifact, is convinced that, if the transference of chromatic substance from the nucleus to the cytoplasm continues during adult life, the process is diffusion and in no sense a transposition of formed chromatic bodies. However, the nuclear origin of the tigroid

substance, at least in the adult, is opposed by VAN DURME, who believes that the tigroid substance is derived from the nucleo-albumins of the lymphocytes.

VAN GEHUCHTEN ('97), accepting the reticular structure of the ground substance, believes that the tigroid substance accumulates at the nodal points of the net. These accumulations enlarge in certain regions and form masses which appear homogeneous when they become sufficiently dense to obscure the reticular framework. A mesh of the reticulum which remains unfilled produces a vacuole. VAN GEHUCHTEN attributes the formation of these bodies to the affinity of the protoplasmic net for a special chemical substance. It is, therefore, the form and disposition of the cytoplasmic net that determines the size and distribution of the tigroid bodies; and this, in turn, governed by the function of the neurone.

is

SCOTT ('99), on the contrary, argues that the shape and distribution of the tigroid bodies are determined by the modification in the shape of the cell during growth. The tigroid substance diffusely permeates the whole cytoplasm in the early history of the cell. But the growth of the cell and especially the development of the protoplasmic processes break the substance up, by a purely mechanical process, into separate bodies. The distribution of these bodies would seem, therefore, to be governed by the proportions and shape assumed by the

cell.

SCOTT takes the position, furthermore, that the tigroid substance cannot be considered as a part of the cytoplasmic net since it diffuses into the cytoplasm after the net is formed. Though the two substances may be intimately associated, they are structurally distinct. He considers that the granules are homogeneous and that appearances to the contrary, including vacuoles, are to be explained by irregularities in their surfaces.

In Lophius HOLMGREN (99) finds that the tigroid body may be resolved into two constituents: a homogeneous ground substance, which stains bluish gray in iron haematoxylin and which tends to stain in both colors with toluidin-erythrosin; and granules which are suspended in the ground substance and which stain black in iron haematoxylin and deep blue in toluidin-erythrosin. The ground substance forms in areas of varying shape and size and the granules may be diffusely distributed through it or massed into solid bodies of different proportions. HOLMGREN does not discover any definite relation of these bodies to a formed ground substance of the cytoplasm.

Although the substance occurs in comparatively small amounts in

the nerve cells of Lophius, HOLMGREN describes an interesting feature of its distribution in the cell. Its ground substance has a two-fold arrangement relative to the centrosome; in radii which widen as they approach the periphery of the cell, and in concentric circles around the centrosome. Both the radial and circular bands are more or less irregular in outline. This typical arrangement, however, is often found modified or obscured. The circular bands may overlap each other and those which come nearest the nucleus are more or less diverted from their symmetrical course so as to encircle the nucleus. And on the side of the nucleus nearest the centrosome the tigroid substance collects into a mass of unusual size and becomes involved in the process of transfusion of nuclear substance into the cytoplasm as we have described in the section relating to the nucleus.

HOLMGREN ('00) believes that the distribution of the tigroid bodies is determined by the arrangement of the canaliculi. He has noticed that the tigroid substance of a given cell is most abundant in the region of the most conspicuous canaliculi; also, that it is very abundant in the nerve cells of animals which are characterized by numerous canaliculi. Furthermore, electrical stimulation of the nerve cell is accompanied by an increase in the tigroid substance and also by an expansion of the canaliculi.

Although HOLMGREN'S interpretation regarding the increase in tigroid substance under electrical stimulation of the nerve is open to serious objection, his observation would nevertheless support his thesis that the condition of the tigroid substance is correlated with that of the canaliculi. This idea received further support from the observations of PUGNAT (01) who finds that the canaliculi appear synchronously with the tigroid bodies in the embryonic nerve cell of the chick.

Regarding the function of the tigroid substance, its relation to the ground substance and its embryogenesis point to the same conclusion that it is a nucleoproteid whose kinetic energy is transformed into potential energy by the metabolism of the cell. The distribution of the substance through the cytoplasm may, as SCOTT points out, contribute to a more prompt and rapid metabolism than if the activities were restricted to the nucleoproteids within the nucleus. This interpretation seems to be born out further by the facts of chromatolysis.

II. Chromatolysis. In the strict sense "chromatolysis" applies only to a progressive diminution of the chromatic substance of the cell, but other phenomena which seem to be concomitant with this change will be, for convenience, included in this discussion.