Structural Basis of the Cell

STRUCTURAL BASIS OF THE CELL It has been pointed out in Chapter I. that the ultimate structural basis of the cell is still an open question ; for there is no general agreement as to the configuration of the protoplasmic network, and we do not yet know whether the fibrillar or the alveolar structure is the more fundamental. This question is, however, of minor importance as compared with the microsome-problem, which is, I think, the most fundamental question of cell-morphology, and which is equally pressing whatever view we may hold regarding the configuration of the network.

Are the granules described as " microsomes " accidental and nonessential bodies, produced, it may be, by the coagulating effects of the reagents, as Fischer's experiments suggest ? Or are they normal and constant morphological elements that have a definite significance in the life of the cell ? It is certain that the microsomes are not merely nodes of the network, or optical sections of the threads, as the earlier authors maintained ; for the fibrillae may often be seen to consist of regular rows of granules. Van Beneden gave the first clear description of the microsomes in this regard in the following words : " I have often had occasion to note facts that establish the essential identity of the moniliform fibrillae and the homogeneous fibrillae of the protoplasm. In my opinion every fibrilla, though it appear under the microscope as a simple line devoid of varicosities, is formed at the expense of a moniliform fibril composed of microsomes connected with one another by segments of uniting fibrils." I Again, in a later work he says of the fibrils of the astral system in Ascaris " It is easy to see that the achromatic fibrils are moniliform, that they are formed of microsomes united by Similar observations have been made by many later writers. In the eggs of sea-urchins and annelids, which I have carefully studied, there is no doubt that after some reagents, e.g. sublimate-acetic, picroacetic, chromo-formic, the entire astral system has exactly the structure described by Van Beneden in Ascaris. Although the basal part of the astral ray appears like a continuous fibre, its distal part may be resolved into a single series of microsomes, like a string of beads, which passes insensibly into the cytoreticulum. The latter is composed of irregular rows of distinct granules which stain intensely blue with hematoxylin, while the substance in which they are embedded, left unstained by hematoxylin, is colored by red acid aniline dyes, such as Congo red or acid fuchsin.

The difficulty is to determine whether this appearance represents the normal structure or is produced by a coagulation and partial disorganization of the threads through the action of the reagents. A justifiable scepticism exists in regard to this point ; for it is perfectly certain that such coagulation-effects actually occur in the proteids of the cell-substance, and that some of the granules there observed have such an origin. It is very difficult to determine this point in the case of the cyto-microsomes, owing to their extreme minuteness. The

question must, therefore, be approached indirectly by way of an examination of the nucleus and its relation to the cytoplasm. Here we find ourselves on more certain ground and are able to make an analysis that in a certain measure justifies the hypothesis that the cytomicrosomes may be true morphological elements having the power of growth and division like the cell-organs formed by their aggregation.

Nucleus and Cytoplasm

From the time of the earlier writings of Frommann ('65, '67), Arnold ('67), Heitzmann ('73), and Klein ('78), down to the present, an increasing number of observers have held that the nuclear reticulum is to be conceived as a modification of the same structural basis as that which forms the cytoplasm. The latest researches indicate, indeed, that true chromatin (nuclein) is confined to the nucleus.' But the whole weight of the evidence now goes to show that the linin-network is of the same nature, both chemically and physically, as the cyto-reticulum, and that the achromatic nuclear membrane is formed as a condensation of the same substance. Many investigators, among whom may be named Frommann, Leydig, Klein, Van Beneden, and Reinke, have described the threads of both the intraand extra-nuclear network as terminating in the nuclear membrane ; and the membrane itself is described by these and other observers as being itself reticular in structure, and by some (Van Beneden) as consisting of closely crowded microsomes arranged in a network. The clearest evidence is, however, afforded by the origin of the spindle-fibres in mitotic division ; for it is now well established that these may be formed either inside or outside the nucleus, and there is a pretty general agreement among cytologists, with the important exception of Boveri, that both spindle-fibres and astral rays arise by a direct rearrangement of the pre-existing At the close of mitosis the central portion of the spindle appears always to give rise to a portion of the cytoplasm lying between the daughter-nuclei ; and in the division of the egg in the sea-urchin I have obtained strong evidence that the spindle-fibres are directly resolved into a portion of the general reticulum. These fibres are in this case formed inside the nucleus from the linin-network ; and we have therefore proof positive of a direct genetic continuity between the latter and the cytoplasmic structures. But more than this, I have found reason to conclude that in this case a considerable part of the linin-network is derived from the chromatin, that the entire nuclear reticulum is a continuous structure, and that it is no more than a specially differentiated area of the general cell-network ('95, 2). This conclusion finds, I believe, a very strong support in the studies of Van Beneden, Heidenhain, and Reinke reviewed beyond (p. 223) ; but the bearing of these only becomes plain after considering the morphological differentiations of the nuclear network and its transformations during mitosis.