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

SOME PROBLEMS OP REPRODUCTION. 9
seem hardly more than colonies of Phytomastigopods in the
resting state, such as Aigse Confervoidese, and such apocytial
forms as Cladophorese and many Siphonese, in
these I say the gametes differ in no appreciable respect from
the ordinary swarmers or zoospores, save that they are often
smaller; they are usually formed by the segmentation or
rapidly-repeated binary fission of the cell-body of the
gametogonium. In some cases these isogametes, failing
conjugation, may develop like ordinary zoospores; they are
facultative 1 gametes; but in most cases they have lost this
power of independent growth, and are obligatory gametes.
In some cases all the vegetative cells assume the character
and function of gametogonia; in other cases we may distinguish
clearly between " brood cells " (gametogonia or zoosporangia)
on the one hand, and " colonial" or vegetative cells
on the other.
Apart from other differentiations we may discriminate two
grades of isogamy, which we shall term (a) EUISOGAMY, (b)
ExOISOGAMY.
(a) In EUISOGAMY, each gamete has the power of uniting
with the other, irrespective of its origin ; nay, in some cases,
as in Pediastrese, the gametes of a single gametogonium
habitually conjugate with one another, thus forming endogatnous
unions.
(b) In EXOISOGAMY, a gamete will not conjugate with another
of the same brood, but will only mate with one from a different
gametogonium at least. This phenomenon would be difficult
to demonstrate in the free Flagellates; but it occurs in some
of the lowest Confervoidp, such as Ulothrix. In this genus
the gametes, facultative though they be, are strictly exogamous.
In the Vjjlvocine Pandorina the gametes of different broods
vary in size, and the small and middle-sized ones will pair indifferently
with one another, quite independently of their size,
but on the condition that the two gametes belong to distinct
broods. About the largest gametes there is some doubt.
1 I believe De Bary first introduced tbe terms ' facullative' and 'obligatory
' in treating of parasitic fungi, &o.