Davis. — Nuclear Studies on Pellia . 
149 
on opposite sides of the nucleus, forming two caps extending 
over the ends of that structure. The kinoplasm composing 
the caps is at first granular, but finally the greater part of 
the substance is changed into fibrillae. 
It has not been possible to establish centrosomes either 
giving rise to or accompanying the activities of the kinoplasm, 
and the close study of the development of the spindle, together 
with the absence of the structure known as the aster, has led 
to a retreat from the views very generally held previous to 
1896 respecting the presence of centrosomes in the cells 
of Pteridophytes and Spermatophytes. It is probably the 
belief of most plant-cytologists that neither the centrosome 
nor the aster is present in the cells of vegetative tissue in 
these plants or in that phase of ontogeny when the sporophyte 
passes over to the gametophyte. 
The most interesting problem in spindle-formation, apart 
from the very difficult physiological explanation of the 
behaviour of fibrillae, is perhaps the question of phylogenetic 
evolution. How is the condition in the Pteridophytes and 
Spermatophytes, where the fibrillae are largely free and 
distinct from one another and acting independently of a fixed 
centre, to be related to centrosomes and asters present in 
certain Thallophytes ? The problem is one of very broad 
interest, as it involves a form of protoplasmic differentiation 
apparently restricted to the higher plants, and not found in 
the animal kingdom. 
Investigations among the Thallophytes have proceeded 
sufficiently far to establish the presence of beautiful asters for 
the several groups, in certain instances with definite centro- 
somes. The most notable examples are presented by the 
Phaeophyceae in Fucus (Farmer and Williams, ’96 and ’98, 
Strasburger, ’97) ; Sphacelaria , studied with the greatest 
detail by Swingle (’97), and Dictyota (Mottier, ’98 and ’00). 
From the accounts of Lauterborn, ’93 and ’98, we may expect 
these structures in the Diatoms, but other groups of Algae 
are less promising, although they have been very little studied. 
Corallina , a member of the Rhodophyceae (Davis, *98), has 
