ABSTRACT
When a dividing echinoderm egg, in the metaphase of mitosis, is subjected to the pressure of the surface tension existing between a cover slip and a thin film of water, the entire mitotic figure, consisting of a highly viscous jelly cortex which encloses the dilute polar regions and the spindle, suddenly collapses, leaving not a vestige of the structural features of the preceding karyokinetic figure. This collapse is analogous to the sudden breakdown of certain inorganic gels, namely, of iron oxide, as described by Schalek and Szegvary, and of metallic cadmium, as described by Svedberg. In all three cases the sudden liquefaction is brought on by mechanical disturbance, in protoplasm by pressure, and in the two inorganic gels by stirring or shaking. These analogous phenomena tend to support the micellar hypothesis of the structure of gels, a structure in which the colloidal units are connected one to the other to form a three - dimensional network.
The viscosity values given here are based primarily on observations made by the writer on the eggs of the sea-urchin, Tripneustes, in .Jamaica, B.W. I. These data support, in the main, the earlier ones of Chambers 3 on the Echinarachnius egg. More detailed observations on the regional changes in viscosity of the proto-plasm of the dividing echinoderm egg, is to be found in recent publications by Chambers.4, 6
The comparison of the viscosity of the inner protoplasm of an echinoderm egg to that of concentrated glycerin made as a result of microdissection observatiohs 17 was nicely corroborated by comparison of the rate of travel of an 18 μ nickel particle, attracted by a magnet, through the central protoplasm of an egg and through glycerin, the rate in both cases being approximately the same.
Chambers4 has been able to distinguish viscosity differences within the cortical layer during the metaphase of division. He finds the outer region of the peripheral jelly to be of lower consistency than the protoplasmic wedges which form the inter ray substance of the ampbiasters.
The writer bas previously attempted 17 to give some idea of the actual viscosity values of protoplasm by establishing an arbitrary scale of viscosity values, and by comparisons to gelatin solutions and other common substances. It should be remembered that these comparisons are relatively crude, but they give at least an approximate idea of the actual value of the consistency of protoplasm under various physiological conditions. Vague expressions, such as “slightly viscous,” “more viscous,” “liquid” (which may mean any consistency from that of ether to that of soft tar), “non-viscous” (an impossible expression), and number of turns of a centrifuge handle, all suffice, perhaps, for purely relative values of consistency, but give no idea of the actual viscosity of the protoplasm.
These observations on viscosity changes in the dividing echinoderm egg have been determined by the microdissection method. The method consists in the mechanical manipulation of exceedingly fine glass needles under very precise control. The needles are held in a microdissection instrument of which there are three well-recognised types ; the original Barber pipette holder,1 the Chambers microdissection instrument 8 ; and the Péterfi (Zeiss) micromanipulator.16
The criteria used in determining viscosity values of protoplasm by the micro dissection method are, the distance from a moving needle at which particles (microsomes) are disturbed, and the rate at which the protoplasm flows in behind a moving needle. Certain other methods of determining viscosity values or protoplasm, such as the centrifuge method of Heilbrunn 19 and of Weber,22 and the electro-magnetic method of Heilbronn,11 and of Freundlich and Seifriz,10,18 offer means of obtaining statistically precise values of the viscosity of protoplasm provided the type of protoplasm worked upon is such as to permit the use of the various methods.
The microdissection method of making viscosity determinations of protoplasm has certain decided advantages which make it peculiarly suited for ascertaining the consistency of the protoplasm of a dividing egg. One must fully realise that the dividing egg is not a homogeneous mass of protoplasm of which a single viscosity value can be given, but an intricate structure with marked localised differences in consistency. To determine these viscosity values—for example, of living chromo-somes9 —could not possibly be done by any method other than that of microdissection. Failure on the part of some workers to appreciate the presence of even the grosser regional differences in viscosity of the dividing egg, has led to some confusion and misinterpretation of experimental facts.
The. Symbol “µ.µ.,” commonly used to designate 10−9 meter, is misleading. Aµ. (micron) is a millionth part of a meter; a µ.µ., therefore, is a millionth part of a millionth part of a meter, i.e. 10−11 meter. The correct symbol for 10−9 meter (a µ.µ. as commonly used) is “mµ,” i.e. a thousandth of a millionth or a meter.
The term “coagulation” is also misleading when used to designate increased consistency in protoplasm. If living protoplasm ever coagulates it is certainly limited to such extreme cases as the change .which protoplasm undergoes in a protozoan when it encysts, or in a seed when preparing for the winter’s rest. It is possible that such protoplasm is a coagulum, an “irreversible” gel, which reverses into the physiologically active protoplasmic jelly through enzymatic activity at germination: There is no reason to believe that living protoplasm coagulates when in its physiologically active state. With the possible exception of the above cited examples, coagulation is ordinarily a death process.
See in this connection the interesting discussion on the colloid chemistry of cell division by Spek.19
