in Dictyota dichotomci. 177 
of cells. Further than this no observations were made in the 
seedling plant. A nucleus from an enlarging epidermal cell, 
previous to the division which will cut off the stalk-cell of 
the tetrasporangium, is seen in Fig. 25. Here, too, the centro- 
spheres are well developed at the beginning of the prophase. 
In the light of what we now know in Fucus , certain 
Sphaceleriaceae and Dictyota, it seems evident that the body 
which in these plants has been called a centrosome, is one 
that persists from one cell-generation to another in vegetative 
and certain reproductive cells; it is capable of division, and 
is the centre of radiations that give rise to the karyokinetic 
spindle with its mantle and polar fibres. These facts seem 
to argue that the centrosome is a morphological unit of the 
cell, and by the cytologist who is eager to support this theory 
they may be deemed sufficient, but are they conclusive? 
While I admit that this evidence is the strongest that has, 
as yet, been advanced, nevertheless I am not convinced that 
it is conclusive proof, for there are other questions suggested 
here and in the higher plants which require further elucida¬ 
tion. We may appropriately inquire what the relation existing 
between the centrosome and its surrounding radiations is. 
Are the radiations outgrowths of the centrosome as a primary 
morphological unit or organ? Or are they constructed out 
of a differentiated part of the cytoplasm, say the kinoplasm, 
by the centrosome as a centre of activity? Or, finally, is 
the centrosome only a denser mass of kinoplasm formed by 
the meeting of the polar radiations and spindle-fibres ? 
The answer to these questions will necessarily influence 
our conception of the centrosome and also the significance 
of what we term kinoplasm. 
When certain Algae and Fungi are considered alone, nothing 
is more plausible than that the radiations grow out of the 
centrosome as a primary source. In the higher plants, 
however, where there are no centrosomes, this argument will 
not stand, for there is no good reason to believe that the 
spindle-fibres in the Lily are not homologous with those in 
the Brown Algae or in Fungi. If kinoplasmic fibres which 
