2, MARCUS M. HARTOG. 
D. Tue 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).! The gametes of this 
last species are of three sizes, micro-, meso-, and mega- 
gametes, which we may letter a, b, C respectively. These are 
in the first place strictly exogamous, but subject to this con- 
dition 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, con- 
current with its enlargement, the form C had become too inert 
to form isogamous unions. We might say that and b are 
sexually differentiated with respect to C, but not between them- 
selves or with one another. We may conceive that the game- 
togenic divisions in a species being inconstant, broods of 
gametes would be formed whose size was inversely propor- 
tional to the number of the brood ;? 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 
’ The following account is based on the abstract in Biitschli, op. cit., p. 788. 
* In Pandorina, however, each gametogonium forms eight gametes, large, 
medium, or small as the case may be. In Ulothrix 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. 
