The phenomena which have just been described would appear to occur in the segmentation of ova with remarkable constancy and without any very considerable variations.
The division of the ovum constitutes a special case of cell division, and it is important to determine to what extent the phenomena of ordinary cell division are related to those which take place in the division of the ovum. Without attempting a full discussion of the subject I will confine myself to a few remarks suggested by the observations of Flemming, Peremeschko and Klein. The observations of these authors shew that in the course of the division of nuclei in the salamander, newt, etc. the nuclear reticulum undergoes a series of peculiar changes of form, and after the membrane of the nucleus has vanished divides into two masses. The masses form the basis for the new nuclei, and become reconverted into an ordinary nuclear reticulum after repeating, in the reverse order, the changes of form undergone by the reticulum previous to its division.
It is clear without further explanation that the conversion of the nuclear reticulum of the segmentation nucleus into the striæ of the spindle is a special case of the same phenomenon as that first described by Flemming in the salamander. There are however some considerable differences. In the first place the fibres in the salamander do not, according to Flemming, unite in the middle line, though they appear to do so in the newt. This clearly cannot be regarded as a fact of great importance; nor can the existence of the central thickenings of the striæ (nuclear plate), constant as it is for the division of the nucleus of the ovum, be considered as constituting a fundamental difference between the two cases. More important is the fact that the striæ in the case of the ovum do not appear, at any rate have not been shewn, to form themselves again into a nuclear network.
With reference to the last point it is however to be borne in mind (1) that the gradual travelling outwards of the two halves of the nuclear plate is up to a certain point a repetition, in the reverse order, of the mode of formation of the striæ of the spindle, since the striæ first appeared at the poles and gradually grew towards the middle of the spindle; (2) that there is still considerable doubt as to how the vesicular bodies formed out of the nuclear plate reconstitute themselves into a nucleus.
The layer of clear protoplasm around the nucleus during its division has its homologue in the case of the division of the nuclei of the salamander, and the rays starting from this are also found. Klein has suggested that the extra-nuclear rays of the stars around the poles of the nucleus are derived from a metamorphosis of the extra-nuclear reticulum, which he believes to be continuous with the intra-nuclear reticulum.
The delicate connective filaments usually visible between the two halves of the nuclear plate would seem from Strasburger’s latest observations (No. [104]) to be derived from the nuclear substance between the striæ of the spindle, and to become eventually reabsorbed into the newly-formed nuclei.
We are it appears to me still in complete ignorance as to the physical causes of segmentation. The view that the nucleus is a single centre of attraction, and that by its division the centre of attraction becomes double and thereby causes division, appears to be quite untenable. The description already given of the phenomena of segmentation is in itself sufficient to refute this view. Nor is it in the least proved by the fact (shewn by Hallez) that the plane of division of the cell always bears a definite relation to the direction of the axis of the nucleus.
The arguments by which Kleinenberg ([93]) attempted to demonstrate that cell division was a phenomenon caused by alterations in the molecular cohesion of the protoplasm of the ovum still in my opinion hold good, but recent discoveries as to the changes which take place in the nucleus during division probably indicate that the molecular changes which take place in the cohesion of the protoplasm are closely related to, and possibly caused by, those in the nucleus. These alterations of cohesion are produced by a series of molecular changes, the external indications of which are to be found in the visible alterations in the constitution of the body of the cell and of the nucleus prior to division.
Bibliography.
In addition to the papers cited in the last Chapter, vide