THE HISTOLOGY OF THE BLOOD OF LARVA OF LEPIDOSIREN PARADOXA. 301 
‘between the centres, the result necessarily is to cause the centres to separate from one 
another. 
Thus the separation of the centrosomes cannot here be due to the growth of a central 
spindle between them, but more probably to the contraction or retraction of the astral 
rays on their outer sides. 
When the centrosomes have drawn apart to such a distance that the forces of which 
each is the expression can dominate half the cell, the whole protoplasm arranges itself 
symmetrically round them, and a position of equipoise is reached. The cytoplasm is 
already divided into two exactly equal portions, with a neutral zone between. 
When the nuclear membrane disappears on the side next the spindle, it is noticed 
that the spindle system is disproportioned. ‘The greater development on the nuclear 
side is possibly due to the taking up by the two centres of nuclear substance, rather 
than to an increase in the growth of the astral rays on that side, as has been suggested. 
It has been a point much discussed, how far the nuclear substance shares in the for- 
mation of the spindle system. In this case, while the system is formed apparently 
wholly in the cytoplasm, it seems almost certain that the achromatic substance of the 
nucleus is also drawn into and divided in it. 
The arrangement of the spindle fibres in the anaphases is much like that of the outer 
polar fibres in Boverr’s figures of the dividing eggs of Ascaris,* but there is no plate at 
the equator. That the chromosomes are separated by a pushing force on the central 
spindle is excluded here by the absence of a developed central mass of fibrils. 
The subequatorial fibres become exposed on the separation of the daughter chromo- 
somes, and | believe they are related here to the division of the cell body, not in virtue 
| of a pushing or expansive force, as Mrves described,t but of a contractive force. In 
fig. 15, Pl. II. the condition of things is pretty clear. In the axis of the spindle system 
there is a very loosely arranged mass of fibres, with large spaces between the threads, 
while peripherally, from under the reconstructing nuclei the subequatorial fibres extend 
towards the equator, and are there continuous with the cell membrane. ‘The threads 
from the opposite poles meet exactly at the equator on the surface of the cell. At this 
stage the spindle poles are separated from one another, whether by a determination of 
the enchylema, or substance from the contracting daughter nuclei, to the equator or 
otherwise, and the consequence must be to put the longest subequatorial threads, 7.e. 
those reaching the surface at the equator, on the stretch, and if they be of a colloid 
nature, they will, by their elastic tension, tend to retract on to their centres. 
Thus we have produced a disposition of the protoplasmic threads, which is roughly 
indicated in a rude model which I have constructed (text-fig. 1). It is an indiarubber 
balloon, with a band applied round the equator, to which threads are attached. The 
threads are brought out through tubes, the inner orifices of which are carried some 
distance into the interior. When the balloon is inflated through one of the tubes with 
the threads loose, the result is such as represented in fig. 1; when they are drawn tight 
* Zellen-Studien, Helt 2, 1888. + Arch. f. Entwickelungsmek., Bd. v. 
