44 INTRODUCTION. 
of the new cell-wall is gradual and progressive from the old cell-wall 
inward, instead of being developed simultaneously from a plasma 
membrane previously formed. Whether in such cases new. plasma 
membranes are formed across the ends of the daughter cells which come 
in contact with the new transverse cell-wall the author is unable to 
state. 
THE CENTROSOME AND THE BLEPHAROPLAST. 
As illustrations of karyokinesis in which the spindle arises through the 
agency of centrospheres I have selected the tetraspore mother-cell of Déc- 
¢yota and the ascus of certain Ascomycetes, because the centrosphere 
is probably best known in those cells and because the entire develop- 
ment of the mitotic figure has been followed in great detail. In these 
plants, as well as in Mucus and certain Sphacelariacee, we have seen 
that the body which we call a centrosome is one that persists from one 
cell-generation, or nuclear generation, to another in vegetative and in 
certain reproductive cells. It seems to be capable of division, and is 
the centre of radiations that give rise to the karyokinetic spindle. We 
do not know with absolute certainty that the centrosome divides, 
although the evidence seems to admit of no other interpretation. 
In addition to the plants just mentioned, centrospheres have been 
found in some liverworts, in diatoms, and in certain Rhodophycee. 
In the diatoms, however, the behavior of the centrosome during karyo- 
kinesis, as described by Lauterborn (’96), differs widely from the 
typical cases described in the preceding pages. In species of Pinnu- 
laria, Surtrella, and others, Lauterborn finds that the peculiar cen- 
tral spindle arises from the centrosome by a division or process of 
budding. ‘‘Es scheint mir keinem Zweifel zu unterliegen, dass die 
Anlage der Centralspindel aus dém Centrosom durch eine Theilung 
(oder, wenn man lieber will, Knospung) hervorgeht” (1. c., p. 61). 
In the diatoms in question the original centrosome is a relatively 
large globular body which is the center of a system of beautiful radia- 
tions. Soon after the budding off of the primordium of the central 
spindle, the original centrosome, with its radiations, disappears, and 
what is taken to be the new centrosomes arise near the poles of the 
spindle and apparently from it. 
So far as the author is aware, such a phenomenon has no parallel 
among plants, and it is impossible to bring the process of spindle- 
formation in the diatoms, as described by Lauterborn, into line with 
anything known in other organisms. 
When we consider the facts alone in the alge and fungi mentioned, 
we certainly have strong evidence in favor of the doctrine of the genetic 
