418 Graham.—Centrosomes in Fertilization Stages 
In the present paper I shall describe only the stages after the egg has 
been penetrated by the antherozoid, when the pronuclei are already near 
together or in contact (PI. X, Figs, i, 3 ). During these stages the cytoplasm of 
the egg of Preissia quadrata is plainly made up of two zones. The inner 
zone is granular, with rounded bodies forming a dense aggregate that lies in 
masses around the pronuclei and among the rays of the centrospheres. A 
small amount of the same material also clings to the cytoplasmic fibres at 
the periphery of the cell (Figs. 1 , 3 , 4 ). This dense cytoplasm may appear 
more or less alveolar at this stage. Prior to fertilization the whole cytoplasm 
of the egg has this consistency and is quite dense. The outer zone of the 
cytoplasm is coarsely vacuolar. The films between the vacuoles are very 
thin and delicate. A few larger and quite dense homogeneous granules are 
scattered through both zones of the cytoplasm. Between the nucleus of the 
antherozoid and the egg nucleus lies a small mass or body unlike the cyto¬ 
plasm just described. From its position in close proximity to the nucleus 
of the antherozoid, the small quantity visible, and the absence of similar 
cytoplasm anywhere else in the cell, it might be thought to be cytoplasm 
brought in by the antherozoid, but I have no proof that this is the case. 
The egg and male nuclei are plainly differentiated by their size and in 
Fig. 3 lie in the central part of the cell, separated by a very little cytoplasm. 
In Fig. 1 their membranes are in contact. 
In the cytoplasm at the opposite poles of the egg nucleus astral rays 
are seen converging upon small, dense, rounded bodies, the centrosomes 
(Figs. 1 , 3 , 4 , 6 ). These rays extend long distances through the cytoplasmic 
ground substance. They may pass close to the nuclear membrane or 
may touch it. A fibre radiating from the centrosome at the upper part 
of Fig. 1 touches the nuclear membrane of the antherozoid ; another 
radiating from the same centrosome touches the outer membrane of the egg 
nucleus. These astral rays make up an open aster with few rays; but 
they are very definite, coarse, granular fibres, easily distinguishable from 
other cytoplasmic fibres. Peripherally they end rather abruptly and have 
no conspicuous physical connexion with any of the other elements of the 
dense cytoplasm. It is quite possible that there are other shorter and more 
delicate rays, but I have drawn only those which are plainly differentiated. 
In the fewness of their rays these asters resemble those described and 
figured by Farmer in the spore mother-cells of Pellia epiphylla . Figs. 4 
and 6 represent two sections of an egg-cell on either side of the nucleus. 
The centrosphere shows advantageously in Fig. 4 , in which some of the 
definite, coarse astral rays lie over the nuclear membrane, which is almost 
free of all other cytoplasmic material, while others extend into the dense 
alveolar cytoplasm. All these fibres centre on a centrosome, which seems 
to be a single body. In the section on the other side of the nucleus the 
centrosphere lies farther away from the nucleus, all of the astral rays running 
