Centrosomes in Fertilization Stages of Preissia 
quadrata (Scop.), Nees. 
BY 
MARGARET GRAHAM. 
With Plate X. 
S O far as the evidence in the literature on the subject goes, it seems clear 
that while centrosomes are present in the nuclear division figures of many 
algae and fungi, they are just as regularly absent from nuclear division figures 
in the higher plants. In both, however, they are active as blepharoplasts in 
connexion with the formation of cilia. There is very little evidence as to 
whether centrosomes are present during the stages of fertilization in plants, 
though in animals it is fairly well established that, in many cases at least, 
the centre which has been brought into the egg by the sperm divides in the 
formation of the cleavage spindle. In the Bryophytes and Pteridophytes, 
in a number of cases, the centre which becomes the blepharoplast is 
described as arising by division from a centre which has appeared at the 
poles of the karyokinetic figure in the last preceding division or in several 
of the preceding nuclear divisions. No one has, however, followed the 
centre through the processes of fertilization in these forms. As is described 
below, I have been able to demonstrate that in Preissia quadrata centres are 
present at the time when the pronuclei come together. 
It is not to be forgotten that evidence has accumulated that structures 
at least closely simulating centrosomes in appearance can be produced 
artificially in the cytoplasm of eggs stimulated with chemicals. 
As a result of stimulating the egg-cells of Strongylocentrotus lividus at 
various stages, Hertwig (1887) observed artificial astrospheres in the cyto¬ 
plasm, sometimes isolated and sometimes associated with chromosomes. 
The astral rays of these artificial astrospheres of Hertwig are said to radiate 
from a clear area in which there is no centrosome. 
By stimulating the eggs with salt solutions Morgan (1899) induced 
astrosphere-formation also in the cytoplasm of the unfertilized eggs of 
Arbacia. These artificial astrospheres of Morgan centre on a dense body. 
Morgan also observed many astrospheres with clear centres similar to those 
[Annals of Botany, Vol. XXXII. No. CXXVII. July, 1918.] 
