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_ (101) W. Flemming. "Beiträge z. Kenntniss d. Zelle u. ihrer Lebenserscheinungen." _Archiv f. mikr. Anat._, Vol. XVI., 1878. (102) E. Klein. "Observations on the glandular epithelium and division of nuclei in the skin of the Newt." _Quart. J. of Micr. Science_, Vol. XIX., 1879. (103) Peremeschko. "Ueber d. Theilung d. thierischen Zellen." _Archiv f. mikr. Anat._, Vol. XVI., 1878. (104) E. Strasburger. "Ueber ein z. Demonstration geeignetes Zelltheilungs-Object." _Sitz. d. Jenaischen Gesell. f. Med. u. Naturwiss._, July 18, 1879. _External Features of Segmentation._ [FIG. 38. VARIOUS STAGES IN PROCESS OF SEGMENTATION. (After Gegenbaur.)] In the simplest known type of segmentation the ovum first of all divides into two, then four, eight, sixteen, thirty-two, sixty-four, etc. cells (fig. 38). These cells so long as they are fairly large are usually known as segments or spheres. At the close of such a simple segmentation the ovum becomes converted into a sphere composed of segments of a uniform size. These segments usually form a wall (fig. 39, E), one row of cells thick, round a central cavity, which is known as the segmentation cavity or cavity of Von Baer. Such a sphere is known as a blastosphere. The central cavity usually appears very early in the segmentation, in many cases when only four segments are present (fig. 39, B). [FIG. 39. THE SEGMENTATION OF AMPHIOXUX. (Copied from Kowalevsky.)