192 Lloyd Williams. — Studies in the Dictyotaceae • 
fertilization is entirely external as it is in the Fucaceae, that unfertilized 
eggs will pass through a few of the earlier segmentation stages by an 
abnormal process of parthenogenesis, but that normal germlings can only 
be produced from fertilized eggs. 
The following is the method employed for securing material for this 
stage. Oogonial and antheridial plants are collected when the sori are 
quite mature, if possible on the evening preceding the day of maximum 
liberation. The plants are kept separate in moist chambers till the follow- 
ing morning, pieces are then immersed in glass dishes containing clean 
sea- water. The oospheres slowly burst through the walls of the oogonia 
and gradually sink to the bottom of the vessel. Before a sufficient number 
have been liberated to make it profitable to experiment with, half an hour 
or more will have elapsed. When the antheridial plants are immersed the 
antherozoids come out of the restraining cells very quickly, and soon the 
water is cloudy with swarming sperms. If now some of the latter are 
added to the oospheres by means of a pipette, it will be seen that instead of 
impartially surrounding all the eggs, as do Fucus antherozoids, many of the 
eggs are here passed over and entirely ignored, whereas others are covered 
with antherozoids, which in some cases lie three to twelve layers deep. If 
the same material be fixed after an interval of twenty-four hours and sub- 
sequently sectioned it will be found that the antherozoid-covered eggs have 
segmented in a perfectly normal manner, whereas the others show the 
abnormal mode of division described below, a method which is character- 
istic of the so-called parthenogenesis already referred to. 
When pieces of oogonial plants have been immersed for about an hour 
the eggs set free will have been liberated at various intervals of time, some 
nearly an hour, others only a few minutes. If now instead of adding 
antherozoids we test the eggs with distilled water, we find that those that 
have been liberated for half an hour or more have already acquired walls. 
This partly explains why they are no longer capable of attracting the 
antherozoids, and why they cannot possibly be fertilized. From this cir- 
cumstance it is very difficult to get uniformity of stages in cultures of 
Dictyota oospheres. 
Although the mode of fertilization in Dictyota is very similar to that 
in Fucus there are important differences. In the former the liberation of 
the gametes is simultaneous ; this tends to give it a great advantage over 
Fucus , where there is no regular periodicity in the production of the sexual 
cells. On the other hand, the eggs in Dictyota very soon lose their 
capacity for fertilization, whereas those of Fucus may retain it for days. 
This would seem to act as a disability, and so to neutralize the advantage 
obtained by the simultaneous emission of gametes. 
As already explained it is my intention to deal with physiological con- 
siderations in a succeeding paper ; two points, however, demand mention here. 
