OVUM.
Heidenhain ('94, '95) has recently developed this conception of polarity in a very elaborate manner, maintaining that all the structures of the cell have a definite relation to the primary axis, and that this relation is determined by conditions of tension in the astral rays focussed at the centrosome. On this basis he endeavours to explain the position and movements of the nucleus, the succession of divisionplanes, and many related phenomena. In the present state of the subject, Heidenhain's theories must be regarded as somewhat transcendental, though they give many suggestions for further investigation.
Hatschek ('88) and Rabl ('89, '92), on the other hand, have advanced a quite different hypothesis based on physiological considerations. By " cell-polarity " these authors mean, not a predetermined morphological arrangement of parts in the cell, but a polar differentiation of the cell-substance arising secondarily through adaptation of the cell to its environment in the tissues, and having no necessary relation to the polarity of Van Beneden. (Fig. 17, B, C.) This is typically shown in epithelium, which, as Kolliker and Hackel long since pointed out, is to be regarded, both ontogenetically and phylogenetically, as the most primitive form of tissue. The free and basal ends of the cells here differ widely in relation to the foodsupply, and show a corresponding structural differentiation. In such cells the nucleus usually lies nearer the basal end, towards the source of food, while differentiated products of the cell-activity are formed either at the free end (cuticular structures, cilia, pigment, zyrnogengranules), or at the basal end (muscle-fibres, nerve-fibres). In the non-epithelial tissues the polarity may be lost, though traces of it are often shown as a survival of the epithelial arrangement of the embryonic stages.
But, although this conception of polarity has an entirely different point of departure from Van Beneden's, it leads, in some cases at least, to the same result ; for the cell-axis, as thus determined, may coincide with the morphological axis as determined by the position of the centrosome. This is the case, for example, with both the spermatozoon and the ovum ; for the morphological axis in both is also the physiological axis about which the cytoplasmic differentiations are grouped. Moreover, the observations of Heidenhain, Lebrun, and Kostanecki indicate that the same is true in epithelium ; for, according to these authors, the centrosome is always situated on that side of the nucleus turned towards the free end of the cell. How far this law holds good remains to be seen, and, until the facts have been further investigated, it is impossible to frame a consistent hypothesis of cell-polarity. The facts observed in epithelial cells, are, however, of great significance ; for the position of the centrosome, and hence the direction of the axis, is here obviously related to the cell-environment, and it is difficult to avoid the conclusion that the latter must be the determining condition to which the intracellular relations conform. When applied to the germ-cells, this conclusion becomes of high interest ; for the polarity of the egg is one of the primary conditions of development, and we have here, as I believe, a clue to its origin.'