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

SOME PROBLEMS OF REPRODUCTION. 11
growth failing conjugation, though the microgametes, indeed,
are said to form but weakly plants. We know of no other
case where a well-differentiated male cell retains this power
of independent growth or parthenogenesis in the strictest
sense.
(4) OOGAMY.—This term has been freely applied to cases
of anisQgamy and hypoogamy. It is, however, better restricted
to those cases in which the megagamete is neither ciliate nor
flagellate, but motionless, or at the outside slightly amoeboid;
while the microgamete or spermatozoon is most frequently a
free-swimming cell, and consequently retains its primitive
mastigopod form in the majority of cases, including the highest
Metazoa. The megagamete is usually termed an ovum or egg
in animals; but there are profound differences, morphological
and physiological, between the immature metazoon egg as a
progameteandtheegg after the expulsion of the polar bodies
as a true gamete; and I shall henceforth designate the egg
in this latter stage an "oogamete" or "oosphere," reserving the
words "egg" and "ovum" loosely for all stages.
In most cases of oogamy the microgamete is very minute,
reduced to a " resting" nucleus, with just enough cytoplasm
to cover it and carry it up to the oosphere. This reduction in
size finds a curious parallel in the reduction of the male
Rotifer, a bag with sexual organs, and just enough other
organs to enable him to find and fertilise the female, the organs
of nutrition being completely absent.
The lowest oogamous groups are certain Volvocinere
belonging to Phytomastigopods, and the Confervoid genera
(Edogonium and Cylindrocapsa.
(5) SIPHONOGAMY,—Yet another mode of reproduction has
to be noted, combined with any of the preceding, that where
the gametes reach and unite, not by ordinary locomotion and
as naked cells, but by a protoplasmic outgrowth of the gamete
or gametogonium, protected by a cellulose tube. This is teimed
SIPHONOGAMY by Engler; it is a mere distinction ia the
mechanism of karyogamy, for it is associated with isogamy
in some Conjugatae, anisogamy in others of this group and