CHAPTER IV.—FECUNDATION ; HETEROGAMETES. 
In the preceding chapters we have considered sexual reproduction 
in certain of those Thallophyta in which no very marked differentia- 
‘tion of the gametes has been attained, although in Hctocarpus espe- 
cially, and even in Spzrogyra and Basidiobolus, a tendency toward 
a differentiation into male and female cells is manifested. Nor have 
we found any modification of the cells bearing the gametes into dif- 
ferentiated sexual organs, unless the gametangia of such forms as 
Eictocarpus be so considered, and even then there is no apparent 
- difference between male and female gametangia. As already men- 
tioned in the introductory chapter, the terms made and female sexual 
cells are essentially the expression of a certain fundamental kind of 
division of labor, and in the developmental history of sexuality in 
plants we find this division of labor manifested in the gametes them- 
selves before a corresponding differentiation is apparent in the organs 
bearing them. 
SPH ZZROPLEA. 
Among the alge one of the best known and most interesting exam- 
ples of this fact is illustrated in Spheroplea annulina, To Ferdinand 
Cohn (’55) is due the credit of having established the fact of sexual 
reproduction in this genus, a phenomenon among the alge little known 
at the time. Later Spheroplea was studied by Heinricher (’83), 
Rauwenhoff (’88), Kny (’84) and more recently by Klebahn (’99). 
Although both Heinricher and Rauwenhoff followed the behavior of the 
nucleus during certain stages in the development of the sexual cells and 
in fecundation, yet in many respects their work was incomplete. For 
a more thorough investigation of this process, however, we are indebted 
to the researches of Klebahn, who studied the two varieties of the 
species, S. axnulina var. braunzé (Keutz) Kirchner and S, annulina 
var. crassisepta Heinricher. The chief interest in the sexual. repro- 
duction of this plant centers upon the fact that in var. braunzd several 
nuclei are usually present in the egg-cell. 
The contents of the multinucleate cells of Spheroplea present the 
well-known and characteristic arrangement: In typical cases the cen- 
tral cavity of each cell is traversed by a row of large vacuoles inter- 
spersed by smaller ones of varying size. The protoplasm, which forms 
only a thin layer between the larger vacuoles and the cell-wall, is 
collected into dense ring-like or band-shaped masses between the 
