Some Problems of Reproduction: a Comparative Study of ...

SOME PflOBLEMS OF REPRODUCTION. 13
plasm which form the zoospores or gametes ; and the vesicle
so formed persists and is usually expelled with them. But this
formation does not always take place, and no share is taken
by the nucleus in it. A similar vacuolar bladder is formed in
certain Siphonese. The zoospores vary in number, owing to
the number of nuclear bipartitions that produce them; the
smallest (of several sizes, however) and most numerous conjugating
exogamously as (facultative) isogametes. Dodel-Port
relates 1 that he has seen copulation between small active
swarmers and larger more sluggish ones, though they usually
conjugate with those of the same size. So that we have here a
combination of isogamy and anisogamy.
CYLINDROCAPSA is oogamous. The oosphere is formed
simply by the enlargement of a single cell, and is facultative.
The sperm atogonia are formed by the rapid transverse fissions
of the vegetative cells; and the cell-body of each divides to
form two spermatozoa.
CHLAMYDOMONAS PULVISCULUS, referred to above as anisogamous,
shows a transition to the siphonogamy of the
group we shall next examine, for the gametes come to rest,
surrounded each with a cell-wall, and the microgamete
" creeps " into the cell-chamber of the megagamete to fuse
with it therein.
The DESMIDS, CONJUGATES, and DIATOMS are forms permanently
enclosed in their cell-walls, and destitute of cilia or
flagella, never even forming zoospores. In these conjugation
is altogether isogamous, or in certain Conjugates (Spirogyra)
the male is only distinguishable by its slightly smaller
size, and by passing, like that of Chlamydomonas pulvisculus,
into the megagamete cell-chamber to form the zygote
therein. In other cases siphonogamy also occurs, but the
zygote is formed at the junction of the tubes emitted by the
isogametes. In Diatoms the gametes leave their shells to
conjugate as naked cells.
In the Desmid Closterium lunula and certain Diatoms
1 "Ulothrix zonata," in 'Pringsheim's Jahrbiiclier,' vol. x, 1876,
p. 539.