II. The Cytology of the Gametophyte Generation. 201 
some surrounding itself with a membrane. It is a curious fact that in 
Dictyota a stage like Fig. 5 in the Tetraspore paper (PI. IX) is followed 
by one similar to Fig. 33 of the same series, or Figs. 6, 27 accompanying 
the present paper, where the chromosomes are less crowded than before. 
Whatever be the cause of the former condition, it probably facilitates 
the formation of a common membrane, and so prevents the separation 
of the chromatic elements. 
It has been shown that the thickness of the membrane varies con- 
siderably in different stages : we find it to be thinnest, for instance, during 
the synapsis stage of the tetrasporangium nucleus. This has probably 
some significance from the point of view of nutrition. The further fact 
that at certain periods, in some newly formed nuclei for example, the 
nuclear network contracts away from the membrane far more easily than 
at other stages, also suggests that the relation of the network to the 
membrane, and probably also to the cytoplasm, varies from time to time. 
4. Segmentation in parthenogenetic germlings. Is the separation of the 
nuclei into two groups due to inherent repulsion between the nuclei of 
the separating groups, or is it due entirely to the action of the cytoplasm ? 
If the former, then there must have been some sort of division of the 
chromosomes, or, failing that, subsequent differentiation in the characters 
of the nuclei must have taken place. At present the matter is obscure, 
and must be left over for further investigation. 
Summary. 
1. The sexual cells, unlike the tetraspores, are produced and liberated 
simultaneously in fortnightly crops. Fertilization is external. Eggs 
not fertilized within about half or three-quarters of an hour after liberation 
become invested with walls and germinate parthenogenetically. Freshly 
liberated oospheres strongly attract the antherozoids, are fertilized and 
segment in a normal manner. 
2. The oogonium and antheridium are produced by the increased 
growth of surface cells, which, after cutting off a stalk-cell, form re- 
spectively a single egg, or over 1,500 antherozoids. 
3. There is no division of the nucleus in the oogonium as there 
is in that of Fucns. All the divisions of the antheridium as well as the 
stalk-cell division of the oogonium are homotype, and are very similar 
to the stalk-cell division of the tetraspore, except for the fact that there 
are only sixteen chromosomes. 
4. The antherozoid has the cilium lateral. There may be a second 
very much reduced cilium, but this is difficult to demonstrate. The 
nucleus is in the thicker end of the pear-shaped antherozoid ; the eye- 
spot is very small, and instead of being at the base of the cilium is 
generally near the anterior end of the ‘ beak.’ The antherozoids crowd 
