1917] BOURQUIN—ZYGNEMA 429 
the center, and starch grains which radiate out from the pyrenoid 
(fig. 7). 
Staining differentiates the plastid from the pyrenoid and the 
starch grains. The plastid is stained a bright blue by anilin blue, 
gray by iron hematoxylin, and faintly violet by gentian-violet 
when the starch grains are stained very deeply violet. It is color- 
less when the starch grains are only faintly violet. The latter are 
stained a yellowish red by Magdala red and violet by gentian- 
violet. The pyrenoid stains red with Magdala red and safranin 
and dark blue with iron hematoxylin. The plastid is differentiated 
from the cytoplasm which surrounds it by its structure, the plastid 
appearing homogeneous (figs. 1, 7). 
It is most easily seen in chromatophores which are dividing 
(figs. 17, 18), but any chromatophore in which the starch grains 
are not closely packed shows it. A layer of the substance of the 
plastid always surrounds each starch grain and the pyrenoid, thus 
separating the starch grains from each other and from the pyrenoid. 
This layer can be demonstrated even when the starch grains are 
most closely packed (figs. 1, 2, 3, 10). 
The pyrenoid is a homogeneous structure which stains with 
different intensity in different parts of its mass, so that one part 
of it will be dark blue and another gray, or one part lighter red 
than another. It usually forms a compact, irregularly oval or 
round mass in the center of the plastid from which it is sharply 
differentiated. An examination of the figures will show that it 
may vary greatly from these shapes, but it does not extend up 
between the starch grains as CHMILEWSKIJ (1) believed. Staining 
the chromatophore with Magdala red, which stains the pyrenoid, 
and with anilin blue, which stains the plastid, proves this conclu- 
sively. Occasionally, when there is a large space between two 
starch grains, tongues of the pyrenoid extend a short way up 
between them (fig. 8). These tongues never reach the periphery 
of the plastid, come into direct contact with a starch grain, or 
surround one. 
I have not found more than one pyrenoid in a plastid unless 
that plastid was about to divide, division of the pyrenoid always 
immediately preceding the division of the plastid (figs. 17, 18). 
