CHAPTER II.—FECUNDATION; MOTILE ISO- 
GAMETES. 
ULOTHRIX AND HYDRODICTYON. 
There seems to be no question that the simplest and most primitive 
form of sexuality consists in the union of motile isogametes as found 
among many of the most primitive alge. The chief difference be- 
tween the gametes of such forms as Paxdorina and Ulothrtx, for 
example, and their asexual swarm-spores, from which the gametes 
were undoubtedly derived phylogenetically, seems to be merely phys- 
iological. Generally speaking, the gamete is incapable of developing 
into a normal adult individual. It must unite first with another gamete 
of the same species in order to restore the power of growth and divis- 
ion necessary to the development into an individual common to the 
species, and apart from theoretical considerations (I refer to the num- 
ber of chromosomes which, of course, has not been determined for 
these lower forms) this is the most fundamental distinction made. 
Many other well-known forms among the green alge might have been 
taken as representatives, instead of the two selected, but these have 
been chosen because the development of the reproductive cells from 
the mother-cell has been more carefully worked out here, and because 
the processes in this development are coming to be regarded as more 
important from a genetic standpoint. 
In connection with Ulothrix I have selected Hydrodictyon in order 
to present the cytological processes preparatory to the formation of 
gametes in uninucleate as well as in multinucleated cells. 
The cytological development, leading to the formation of gametes 
and also asexual swarm-spores among the simpler representatives of 
the green alge, has been investigated by a number of earlier observers, 
among whom were Alexander Brown, Cohn, Pringsheim, Dodel, 
Strasburger, Klebs, and lately by Timberlake. 
The well-known and widely distributed UZothrzx consists of a simple 
unbranched filament differentiated into base and apex (Fig. 17, A). 
The cells, except the basal one, which is modified as an organ of 
attachment, are quite alike. Each contains a single nucleus and a 
band-shaped chloroplast in the form of an almost complete hollow 
cylinder. Almost any vegetative cell of the filament save the basal 
one may, without undergoing any external modification, function as a 
gametangium, 
