and the Flagellata . 661 
found that the sexual reproductive process is in essentials 
constant and uniform for the genus, but here it is quite 
different. 
Chlamydomonas affords an unique opportunity of acquiring 
a clear idea of the relation of the conceptions, genus, and 
species at the very foot of the phylogenetic tree, and we shall 
see that characters which are of ordinal and cohortal. value 
very little higher up are here merely of specific value. 
To illustrate this, let us consider the different types of 
sexual reproduction within this single genus. Isolated obser¬ 
vations are found in the works of Ehrenberg (’ 38 ), Stein ( 78 ), 
and others, but the first comparative study of the sexual 
process was made by Goroschankin (’ 91 ). In Chi. Steinii 
and the majority of species the contents of the vegetative 
cell divide by repeated bipartition into sixteen or thirty-two 
small planogametes. These are ciliated ovate bodies, and 
though much smaller than, yet of the same character as the 
vegetative body, except that they have no cell-wall. They 
escape by rupture of the mother-wall, and after swarming 
conjugate in pairs. As they are all of one size the union 
is isogamous, and a round zygote results, which surrounds 
itself with a thick wall and rests. This is a parallel pro¬ 
cedure with that found in the higher Protococcoideae. In 
some other species the planogametes are provided with walls 
when first formed, and thus are quite like vegetative cells 
but for their size. This formation of walled planogametes 
is absolutely unknown in any higher plant than Chlamydo - 
Monas, and even in this genus is only found among the most 
primitive species, i. e. those which begin the vegetative 
division with a longitudinal bipartition. Before these plano¬ 
gametes actually conjugate they must get rid of their cell- 
walls, and for this process we find a continuous series of forms. 
Thus with Chi. longistigma (Fig. 13) the walls of both the 
gametes about to unite are thrown off while the cells are 
still some distance apart; with Chi. reticulata (Fig. 13) one 
gamete seems to throw off its wall before the other, so that 
at conjugation only one quitted wall is to be seen; with 
