468 Smith . — Cytological Studies in the Protococcales. II. 
Intermediate stages between the primary and the completed cleavage to 
form sixteen zoospores are not described. After the zoospores have been 
formed they are discharged through a longitudinal split in the mother-cell 
wall, the innermost layer of which forms a vesicle into which the zoospores 
are extruded. Immediately after their extrusion the zoospores begin 
to move rapidly inside this vesicle. At first they form an irregularly 
shaped mass, but after about fifteen minutes they come to rest forming 
a flat plate of cells. When the movement of the zoospores ceases they are 
slightly emarginate. An hour after, the peripheral cells are still further 
emarginate, and after four hours the emarginations have developed into 
horns. During the first few hours in the life of the colony there are spaces 
between the individual cells, but at the end of twenty-four hours the cells are 
closely applied to one another, and the horns have the same shape as 
in mature cells. The vesicle surrounding the young colony persists for 
a short time only. 
Askenasy (1) is the only one who has studied the cell contents in 
preparations of Pediastrum. He finds that in preparations of P. Boryanum , 
cells 9-13 /x in diameter, there is a single eccentrically located nucleus some 
2 /x in diameter, while cells 13-18 /x in diameter are either uni- or binucleate. 
With the growth of the cell there comes an increase in the number of nuclei, 
so that a large number is present in mature cells. He was unable to study 
nuclear behaviour during the cleavage stages of zoospore formation because 
the entire contents of the cell took the haematoxylin stain so deeply, but he 
assumes that each zoospore contains a single nucleus. Askenasy states 
that the pyrenoid can be recognized in very young cells, and that as the cell 
increases in size there is a corresponding increase in the size of the pyrenoid. 
Surrounding the pyrenoid is the usual ring of starch plates ; there is also 
starch in the cytoplasm. He was unable to follow the history of the 
pyrenoid after the first cleavages in zoospore formation, but found that the 
first cleavage plane either passed through the pyrenoid, dividing it into 
two unequal parts, or left it undisturbed. 
In the youngest cells that I have observed there is always a single 
nucleus and pyrenoid. The nucleus is, as Askenasy ( 1 ) has shown, markedly 
eccentric (PI. XII, Fig. 1), but the pyrenoid has no definite position. In 
spite of its very eccentric position within the cell, the nucleus has no definite 
relation to the margin of the colony. It may be on the side of the cell 
towards the periphery of the colony, on the side towards the centre, or 
it may be lateral (Text- fig. 1). With further cell growth the position of 
the nucleus becomes more central, until at the maximal size for uninucleate 
cells it is frequently central. 
The structure of the nucleus, in spite of its minuteness, is much more 
like that of the nuclei of higher plants than I have found to be the case in 
other Protococcales (12, 13 , 15 ). There is always a single nucleolus and 
