SOME PROBLEMS OF REPRODUCTION. 9 
seem hardly more than colonies of Phytomastigopods in the 
resting state, such as Alge Confervoidez, and such apo- 
cytial forms as Cladophoree and many Siphonez, in 
these I say the gametes differ in no appreciable respect from 
the ordinary swarmers or zoospores, save that they are often 
smaller; they are usually formed by the segmentation or 
rapidly-repeated binary fission of the cell-body of the 
gametogonium. In some cases these isogametes, failing 
conjugation, may develop like ordinary zoospores; they are 
facultative! gametes; but in most cases they have lost this 
power of independent growth, and are obligatory gametes. 
In some cases all the vegetative cells assume the character 
and function of gametogonia; in other cases we may distin- 
guish clearly between “brood cells”? (gametogonia or zoospo- 
rangia) on the one hand, and “ colonial” or vegetative cells 
on the other. 
Apart from other differentiations we may discriminate two 
grades of isogamy, which we shall term (a) Evisocamy, (0) 
Exorsocamy. 
(a) In Hutsocamy, each gamete has the power of uniting 
with the other, irrespective of its origin ; nay, in some cases, 
as in Pediastrew, the gametes of a single gametogonium 
habitually conjugate with one another, thus forming endo- 
gamous unions. 
(6) In Exorsocamy, a gamete will not conjugate with another 
of the same brood, but will only mate with one from a different 
gametogonium at least. This phenomenon would be difficult 
to demonstrate in the free Flagellates; but it occurs in some 
of the lowest Confervoids, such as Ulothrix. In this genus 
the gametes, facultative though they be, are strictly exogamous. 
In the Volvocine Pandorina the gametes of different broods 
vary in size, and the small and middle-sized ones will pair in- 
differently with one another, quite independently of their size, 
but on the condition that the two gametes belong to distinct 
broods. About the largest gametes there is some doubt. 
1 T believe De Bary first introduced the terms ‘ facultative’ and ‘ obliga- 
tory’ in treating of parasitic fungi, &c, 
