THE HAMILTON ASSOCIATION 89 
It is important, however, to remember that the contents of all 
the cells of any given thread in the Spirogyra pass in one direc- 
tion. That is, if the contents of one cell of a given thread pass over 
into a cell of an adjacent thread, then the contents of each cell 
in the first thread will pass in the same direction. It is obvious, 
then, that while there is no structural differentiation apparent, 
there must be a potential differentiation. It is worth noting, too, 
that the contents of one of the cells has acquired the power of 
moving from one place to another. 
In the Ulothrix zonata, a filamentous alga, renroduction is 
effected in two ways, the former of which resembles somewhat 
the reproduction of the unicellular plants; the latter is a simple 
form of sexual reproduction, and this method is the more im- 
portant of the two from the evolutionist’s standpoint. Asexual 
reproduction is effected by means of swarm-spores which have four 
cilia, and are formed by division of any cell in the filament. 
These, after escaping from the cell in which they are formed, give 
rise to new filaments or plants. These swarm-spores, by reason 
of their cilia, have the power of motion. The sexual swarm- 
cells are formed in a similar manner, but in much greater num- 
bers. They are smaller than the swarm-spores and possess only 
two cilia. By conjugation of two swarm-cells a structure is 
formed which passes some time in a resting condition, after which 
a new filament is formed. The excess in number of swarm-cells 
over swarm-spores indicates an increasing tendeney towards 
sexual reproduction. The cilia spoken of are effective organs of 
locomotion, the importance of which will appear later. The fact 
remains, however, that as in the case of the Spirogyra, conjugat- 
ing cells are apparently the same structurally. 
The modes of reproduction just described persist through a 
large number of the lower green plants, usually known as Alge. 
In the sexual mode of reproduction, important modifications take 
place, however, in the direction of differentiation, structurally, 
of the conjugating cells. This differentation in a highly developed 
condition is well exemplified in the Fucus platycarpus, where one 
of the conjugating cells, the egg-cell, has become large relatively 
and merely floats in the water, while the other, the sperm-cell, has 
