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

72 MARCUS M. HABTOG.
D. THE ORIGIN OF SEX.
If we seek for the origin of binary sex we may find a clue
in the history of Ulothrix, or even better the Volvocine
Pandorina, referred to above (p. 9). 1 The gametes of this
last species are of three sizes, micro-, meso-, and megagametes,
which we may letter », b, C respectively. These are
in the first place strictly exogamous, but subject to this condition
the following unions are said to be possible—a+a and
b + b (isogamous), as well as a+b, a+C, b+C (anisogamous): but
the other conceivable pairing,C+C, does not occur; as if, concurrent
with its enlargement, the form C had become too inert
to form isogamous unions. We might say that a and b are
sexually differentiated with respect to C, but not between themselves
or with one another. We may conceive that the gametogenic
divisions in a species being inconstant, broods of
gametes would be formed whose size was inversely proportional
to the number of the brood ; 2 the extreme forms would
be small active gametes and large sluggish ones respectively.
As the latter are ill fitted to conjugate among one another, in
the struggle for pairing the small numerous active ones would
be most likely to find pair with these large ones, and the
rejuvenescence of such unions would be the more efficacious
because of the difference of temperament between the parent
gametes. The middle forms being produced in smaller numbers
than the little gametes, and less useful either way, would tend
to disappear. The difference of size between the micro- and
mega-gametes would tend to increase and a division of labour
take place, the megagamete tending to accumulate nourishment
to give the zygote a good start, the microgamete gaining activity
1 The following account is based on the abstract in Butschli, op. cit., p. 788.
1 In Pandorina, however, each gametogoniutn forms eight gametes, large,
medium, or small as the case may be. In Ulotlirix the number is inversely
as the size : the smallest are capable of isogamous union, which is the rule;
but they are also capable of anisogamous unions with larger, more sluggish
zoospores.