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

8 MARCUS M. HARTOG.
also of karyogamy—the association of nucleus and cytoplast
that are strangers to each other.
"We may fairly adopt the view that multiple isogamy, where
the fusion of the nuclei follows that of the cytoplast, was originally
derived from plasmodial formation, and that binary
isogamy and the higher forms of karyogamy are further stages
in the upward path.
In the AcRASiEiEj the individuals produced by fission simply
aggregate together without fusioD before passing into a resting
state, as a fructification in which their cellular individuality
is retained. The only explanation I can suggest for this is
that it indicates the loss of a primitive formation of plasmodia
at this stage; the Acrasieas would be an apoplasmodial or
apoplastogamous group, a degenerate offshoot of the M y x o m ycetes.
III. THE MODES OF KARYOGAMY AS ILLUSTRATED IN
PROTOPHYTES.
Undoubtedly the lowest forms of life that present us with cases
of karyogamy are the FLAGELLATES ; and in the PHYTOMAS-
TIGOPODS or Green Flagellates we may study its modes from
isogamy to complete sexual differentiation. From the colonial
Phytomastigopods we can trace an almost unbroken line upwards
past the Thallophytes, which in their asexual reproduction
so often revert to the lowly Flagellate type ; up through
the Archegoniata to the Gymnosperms and true Flowering
Plants at the top of the scale. We distinguish the following
modes of karyogamy :—(1) ISOGAMY—(a) EUISSGAMY, (b)
EXOISOGAMY ; (2) ANISOGAMY ; (3) HYPOOGAMY ; (4) OOGAMY.
(1). ISOGAMY.—This is the simplest mode of karyogamy. In
this process cells exactly similar fuse as gametes, cytoplast to
cytoplast, nucleus to nucleus; a single nucleated cell, the
zygote, being the produce. In most cases only two gametes
unite in binary isogamy; more rarely three or more may
fuse in multiple isogamy. In many of the Phytomastigopods,
and some of those simple filamentous or thalloid Algse that