Opalina. 255 
ae 
which seem clearly to be related to true centrosomes are found 
both inside and outside the nuclei. The absence of centrosomes in 
Opalina is apparently not a primitive character. 
In so for as the function of the centrosome is a mechanical 
one, furnishing a point of resistence in the movements attending 
mitosis, it is not needed in the mitosis of Opalina, for the ends of 
the spindle are attached to the nuclear membrane and this membrane 
can furnish the necessary resistence points, if any such be really 
needed. At each constriction of a nucleus in division, both the 
chromatic and the achromatic fibrils at the equator of the nucleus 
are pinched and held by the constricted nuclear membrane, and 
apparently the attachment of at least the chromatic fibrils to each 
end of the nuclear membrane persists even during the resting stage. 
The spindle. 
The mitotic spindle in Opalina is interesting in its simplicity, 
being formed merely by the enlarging of those fibrils and films which 
run lengthwise in the oval nucleus and by the concomitant diminishing 
in size of the transverse fibrils and films, the latter, apparently, for the 
most part, being drawn in like pseudopodia. There is nothing that can 
be interpreted as an outgrowth of fibres from any formative center, as 
seems to occur in connection with the centrosomes in many mitoses.’) 
The mitotic spindle in Opalina is also interesting in the fact 
that it is formed from both chromatic and achromatic material. In 
the resting nucleus the achromatic foam fills the whole nucleus, a 
network of chromatin fibrils being also present over the surface of 
the nucleus just beneath the nuclear membrane. The appearance 
of longitudinal striation in the dividing nucleus is due to the 
emphasizing of the longitudinal strands of the chromatin net and 
the longitudinal films of the achromatic foam. The spindle, therefore, 
is composed of a central achromatic portion and a superficial chromatic 
portion. To what extent the two are connected in either the resting 
or dividing nuclei it is difficult to say.) 
There seems to be little true resemblance between the condition 
in Opalina, with an outer spindle composed of chromatin and an 
inner spindle of achromatic substance, and the condition in many 
") Very similar conditions have been described by Bovert (1887), p. 21) in 
the formation of the spindle in the maturation divisions of Ascaris megalocephala. 
*) Wixson (1895, 1400) believes that the linin which gives rise to the mitotic 
spindle in sea-urchin eggs arises from the chromatin. 
