ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
441 
formation of the equatorial chromatin-plate is solely due to the pressure 
exerted by the two systems of rays from the opposite sides of the nucleus. 
The separation of the equatorial plate into two daughter-plates travelling 
in opposite directions, and the formation of the interzonal filaments are 
due to the continuance of the same action which has been going on 
before — the continuous growth of the achromatic fibrils. When each of 
two daughter-chromatin-plates approach the extremities of the spindle a 
new nuclear membrane is formed around each chromatin-plate, each plate 
thus forming a complete nucleus. The interzonal filaments consist of 
the same substance as the spindle filaments, but they do not in any 
way unite two daughter-chromatin-plates. In the interzonal filaments, 
therefore, there are two systems of filaments which run in opposite 
directions. 
Looking at it in this way, the author considers that the whole 
phenomena of karyokinetic changes may be connected in one continuous 
series of activities of the cytoplasmic asters upon the nucleus. It 
follows that the rapidity of the cleavage process depends, in a great 
measure, upon the rapidity with which the cytoplasmic asters can 
migrate to two opposite poles of the nucleus. The presence, therefore, 
of inert, passive yolk-glanules imbedded in the cell-body of the ovum, 
necessarily interferes with rapid movement of the cytoplasmic asters. 
Such a view of the mechanism of karyokinesis suggests an explanation 
of the well-known fact, that the velocity of cleavage in any part of the 
ovum is, roughly speaking, directly proportional to the concentration of 
the protoplasm, or inversely proportional to the quantity of yolk-granules 
imbedded in the protoplasm. 
The mechanism involved in the multiple nuclear division can be 
explained exactly in the same way as that in the binary karyokinesis. 
If, in a given stage of cleavage, say in the eight-cell stage, one blasto- 
mere on the right-hand side of the bilateral ovum shows multiple 
karyokinesis, the corresponding segment on the left half of the ovum 
shows exactly the same peculiarity. 
j8. Histology.* 
The state in which the Water exists in Live Protoplasm.f— Prof. 
Marcus M. Hartog remarks : — One consideration on the structure of pro- 
toplasm, the question of the mode in which its water is combined with 
it, has been somewhat neglected of recent years. Even Berthold, who 
in his ‘ Protoplasma-mechanik’ J has put forth a masterly exposition of the 
reasons for regarding living protoplasm as an emulsion, seems to have 
overlooked the need of explaining the condition of what may be termed 
the substratum or base of the emulsion in which the droplets lie. 
Yet there is one phenomenon which ever confronts the histologist 
and sheds considerable light on this question, and which, from its very 
obviousness, has hitherto escaped full investigation ; this is the change 
of optical behaviour of the protoplasm after death. Living protoplasm, 
in which I include even such specialized forms as striated muscular fibre, 
is transparent, with a refractive index not far above those of water, cell- 
sap, or the liquids that lave the cavities of the Metazoa. The difference 
* This section is limited to papers relating to Cells and Fibres, 
t Read at the British Association, 1889. 1 Leipzig, 1886. 
1890. 2 I 
