304 ON THE INTERNAL FORM [ch. 



finally only separate dots are seen*. Other sources of error arise from the 

 optical principles concerned in microscopic vision; for the diffraction-pattern 

 which we call the "image" may, under certain circumstances, be very different 

 from the actual object f. Furthermore, the optical properties of living proto- 

 plasm are especially complicated and imperfectly known, as in general those 

 of colloids may be said to be; the minute aggregates of the "disperse phase" 

 of gels produce a scattering action on light,' leading to appearances of turbidity 

 etc., with no other or more real basis J. 



So it comes to pass that some writers have altogether denied the existence 

 in the living cell-protoplasm of a network or alveolar "foam"; others have 

 cast doubts on the main tenets of recent histology regarding nuclear structure ; 

 and Hardy, discussing the structure of certain gland-cells, declared that 

 "there is no evidence that the structure discoverable in the cell-substance of 

 these cells after fixation has any counterpart in the cell when living." "A 

 large part of it" he went on to say "is an artefact. The profound difference 

 in the minute structure of a secretory cell of a mucous gland according to the 

 reagent which is used to fix it would, it seems to me, almost suffice to establish 

 this statement in the absence of other evidence §." 



Nevertheless, histological study proceeds, especially on the part of the 

 morphologists, with but little change in theory or in method, in spite of these 

 and many other warnings. That certain visible structures, nucleus, vacuoles, 

 "attraction-spheres" or centrosomes, etc., are actually present in the living 

 cell we know for certain; and to this class belong the majority of structures 

 with which we are at present concerned. That many other alleged structures 

 are artificial has also been placed beyond a doubt; but where to draw the 

 dividing line we often do not know. 



The following is a brief epitome of the visible changes undergone 

 by a t3rpical cell, subsequent to the resting stage, leading up to the 

 act of segmentation, and constituting the phenomenon of mitosis 

 or caryokinetic division. In the fertilised egg of a sea-urchin we 

 see with almost diagrammatic completeness, in fixed and stained 

 specimens, what is set forth here||. 



* W. S. Duke-Elder, Journ. Physiol, lxviii, pp. 1.54-165, 1930; of. Baurmann, 

 Arch.f. Ophthalm. 1923, 1926; etc. 



t Abbe, Arch.f. mikrosk. Anat. ix, p. 413, 1874; Gesammelte Ahhandl. i, p. 45, 

 1904. 



X Cf. Rayleigh, On the light from the sky, Phil. Mag. (4) xli, p. 107, 1871. 



§ W. B. Hardy, On the structure of cell protoplasm, Journ. Physiol, xxiv, 

 pp. 158-207, 1889; also Hober, Physikalische Chemie der Zelle und der Gewebe, 

 1902; W. Berg, Beitrage zur Theorie der Fixation, etc., Arch.f. mikr. Anat. LXii,' 

 pp. 367-440, 1903, Cf. {int. al.) Flemming, Zellsubstanz, Kern und Zelltheilung, 

 1882, p. 51; etc. 



II My description and diagrams (Figs. 89-93) are mostly based on those of 

 the late Professor E. B. Wilson. 



