204 
METHODS IN PLANT HISTOLOGY 
simultaneous, and at the sixth division, which is the last, there are 
32 mitotic figures, each with 32 chromosomes which split so that 32 
go to each pole. When you can make a preparation in which these 
chromosomes can be counted, your technic is adequate for research 
work in cytology. In a good preparation, the mitotic figures in the 
oogonium show small but brilliant centrosomes, with a great display 
of radiations. (Fig. 49). 
Dictyota .—Didyota deserves a place in the series illustrating the 
evolution of sex, since its large egg has lost all motility, but the 
difference in size of egg and sperm is not so extreme as in Fucus. 
It also furnishes an excellent example of the development of a thallus 
from an apical cell (Fig. 50). 
Mount habit material on paper. For sections, fix in chromo- 
acetic acid. For figures, cut 3 to 5 y, but for general views of apical 
cell and reproductive phases, cut 10 y. Stain in iron-haematoxylin 
and counter-stain for 2 or 3 minutes in safranin. 
