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

10 MARCUS M. HARTOG.
Among the marine Siphonese, Dasycladus evinces a yet
more subtle difference; for Berthold has shown 1 that the
gametes of one brood may refuse to pair, not only with one
another, but also with gametes of certain other plants, while
they do so with others; and yet he failed to make out any
distinguishing character in the plants themselves. 3
(2) ANISOGAMY.—This is the second stage of karyogamy,
where the gametes are similar in form, but of two sizes, large
and small respectively; and in their union a micro- always
pairs with a mega-gamete. This maybe regarded as the
lowest form of sexual differentiation, the smaller more active
microgamete being the male, the larger more passive megagamete
the female. Exogamy is a necessity here, for each
brood is all of one kind, large or small, as the case may be.
If the gametogonia do not show a corresponding difference
of size (which they do sometimes), the differentiation of the
gametes is effected by the smaller number of bipartitions in
the segmentation which produces the megagametes. Thus in
Chlamydomonas pulvisculus the megagametes are produced
in twos or fours, the microgametes in eights.
We shall note below the transitional conditions between
isogamy and anisogamy presented by the genera Ulothrix
and Pandorina.
(3) EhrpooGAMYor HYPERANISOGAMY.—This is our third stage,
recognised but not named by previous writers. The gametes
are similar, but differ not merely in size but in behaviour;
for the megagamete absolutely goes to rest before the microgamete
comes to unite with it. This process occurs in slightly
different modes in two of the lower groups of Olive Seaweeds,
Cutleriacese and Ectocarpese. The Ectocarpese are
remarkable for the fact that their microgametes, as well as the
megagametes, are facultative, or capable of independent
1 'Botanische Zeitung,' 1880, 648. Possibly the explanation is that the
ultimate offspring belonging to the same cycle derived from a single zygote
will not conjugate together any more than they will in certain Ciliate
Infusorians; and that the individuals that showed this mutual sexual
incapacity had this blood relationship.